Delete superversion and log outside of mutex

Summary: As summary. Add two autovectors that get filled up in MakeRoomForWrite and they get deleted outside of mutex

Test Plan: make check

Reviewers: dhruba, haobo, ljin, sdong

Reviewed By: ljin

CC: leveldb

Differential Revision: https://reviews.facebook.net/D18249
kill ReadOptions.prefix and .prefix_seek

Summary:
also add an override option total_order_iteration if you want to use full
iterator with prefix_extractor

Test Plan: make all check

Reviewers: igor, haobo, sdong, yhchiang

Reviewed By: haobo

CC: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D17805
Check PrefixMayMatch on Seek()

Summary:
As a follow-up diff for https://reviews.facebook.net/D17805, add
optimization to check PrefixMayMatch on Seek()

Test Plan: make all check

Reviewers: igor, haobo, sdong, yhchiang, dhruba

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17853
avoid calling FindFile twice in TwoLevelIterator for PlainTable

Summary:
this is to reclaim the regression introduced in
https://reviews.facebook.net/D17853

Test Plan: make all check

Reviewers: igor, haobo, sdong, dhruba, yhchiang

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17985
Read-only BackupEngine

Summary: Read-only BackupEngine can connect to the same backup directory that is already running BackupEngine. That enables some interesting use-cases (i.e. restoring replica from primary's backup directory)

Test Plan: added a unit test

Reviewers: dhruba, haobo, ljin

Reviewed By: ljin

CC: leveldb

Differential Revision: https://reviews.facebook.net/D18297