This commit replaces the use of pthreads in the POSIX port with std::thread
and port::Mutex + port::CondVar. This is intended to simplify porting
the env to a different platform.

The indirect use of pthreads in PosixLogger is replaced with
std:🧵:id(), based on an approach prototyped by @cmumfordx@.

The pthreads dependency in CMakeFiles is not removed, because some C++
standard library implementations must be linked against pthreads for
std::thread use. Figuring out this dependency is left for future work.

Switching away from pthreads also fixes
https://github.com/google/leveldb/issues/381

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=212478311

General cleanup principles:
* Use override when applicable.
* Use const on class members where possible.
* Renames where clarity can be improved.
* Qualify standard library names with std:: when possible, to
  distinguish from POSIX names.
* Qualify POSIX names with the global namespace (::) when possible, to
  distinguish from standard library names.

This also revamps the logic for putting together a message into the
in-memory buffer before that is passed to fwrite(). While correct in
practice, the current implementation advances a char pointer past the
size of its buffer, which is technically undefined behavior.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=211472570

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=192365747

- Replace raw slice comparison with a call to user comparator.
  Added test for custom comparators.

- Fix end of namespace comments.

- Fixed bug in picking inputs for a level-0 compaction.

  When finding overlapping files, the covered range may expand
  as files are added to the input set.  We now correctly expand
  the range when this happens instead of continuing to use the
  old range.  For example, suppose L0 contains files with the
  following ranges:

      F1: a .. d
      F2:    c .. g
      F3:       f .. j

  and the initial compaction target is F3.  We used to search
  for range f..j which yielded {F2,F3}.  However we now expand
  the range as soon as another file is added.  In this case,
  when F2 is added, we expand the range to c..j and restart the
  search.  That picks up file F1 as well.

  This change fixes a bug related to deleted keys showing up
  incorrectly after a compaction as described in Issue 44.

(Sync with upstream @25072954)

- Added DB::CompactRange() method.

  Changed manual compaction code so it breaks up compactions of
  big ranges into smaller compactions.

  Changed the code that pushes the output of memtable compactions
  to higher levels to obey the grandparent constraint: i.e., we
  must never have a single file in level L that overlaps too
  much data in level L+1 (to avoid very expensive L-1 compactions).

  Added code to pretty-print internal keys.

- Fixed bug where we would not detect overlap with files in
  level-0 because we were incorrectly using binary search
  on an array of files with overlapping ranges.

  Added "leveldb.sstables" property that can be used to dump
  all of the sstables and ranges that make up the db state.

- Removing post_write_snapshot support.  Email to leveldb mailing
  list brought up no users, just confusion from one person about
  what it meant.

- Fixing static_cast char to unsigned on BIG_ENDIAN platforms.

  Fixes	Issue 35 and Issue 36.

- Comment clarification to address leveldb Issue 37.

- Change license in posix_logger.h to match other files.

- A build problem where uint32 was used instead of uint32_t.

Sync with upstream @24408625

- Removed one copy of an uncompressed block contents changing
  the signature of Snappy_Uncompress() so it uncompresses into a
  flat array instead of a std::string.
        
  Speeds up readrandom ~10%.

- Instead of a combination of Env/WritableFile, we now have a
  Logger interface that can be easily overridden applications
  that want to supply their own logging.

- Separated out the gcc and Sun Studio parts of atomic_pointer.h
  so we can use 'asm', 'volatile' keywords for Sun Studio.




git-svn-id: https://leveldb.googlecode.com/svn/trunk@39 62dab493-f737-651d-591e-8d6aee1b9529