[Log dumper broken when merge operator is in log]

Summary: $title

Test Plan:
on my dev box

Revert Plan: OK

Task ID: #

Reviewers: emayanke, dhruba, haobo

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D14451
Make an API to get database identity from the IDENTITY file

Summary: This would enable rocksdb users to get the db identity without depending on implementation details(storing that in IDENTITY file)

Test Plan: db/db_test (has identity checks)

Reviewers: dhruba, haobo, igor, kailiu

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D14463
Make GetDbIdentity pure virtual and also implement it for StackableDB, DBWithTTL

Summary: As title

Test Plan: make clean and make

Reviewers: igor

Reviewed By: igor

CC: leveldb

Differential Revision: https://reviews.facebook.net/D14469