Fix GCC -Wshadow warnings in LevelDB's public header files,
reported by Dustin.

Add in-memory Env implementation (helpers/memenv/*).
This enables users to create LevelDB databases in-memory.

Initialize ShardedLRUCache::last_id_ to zero.
This fixes a Valgrind warning.

(Also delete port/sha1_* which were removed upstream some time ago.)