Fix assertion in PosixRandomAccessFile

Summary:
See https://github.com/facebook/rocksdb/issues/244#issuecomment-53372297
Also see this: https://github.com/facebook/rocksdb/blob/master/util/env_posix.cc#L1075

Test Plan: compiles

Reviewers: yhchiang, ljin, sdong

Reviewed By: ljin

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D22419
improve OptimizeForPointLookup()

Summary: also fix HISTORY.md

Test Plan: make all check

Reviewers: sdong, yhchiang, igor

Reviewed By: igor

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D22437
Add is-file-deletions-enabled property

Summary:
Add property 'rocksdb.is-file-deletions-enable'
	 which equals disable_delete_obsole_file_

Test Plan: make all check

Reviewers: sdong

Reviewed By: sdong

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D22119
Don't do memtable lookup in db_impl_readonly if memtables are empty while opening db.

Summary: In DBImpl::Recover method, while loading memtables, also check if memtables are empty. Use this in DBImplReadonly to determine whether to lookup memtable or not.

Test Plan:
db_test
make check all

Reviewers: sdong, yhchiang, ljin, igor

Reviewed By: ljin

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D22281
Use Vector memtable when bulk loading SpatialDB