[RocksDB] Use raw pointer instead of shared pointer when passing Statistics object internally

Summary: liveness of the statistics object is already ensured by the shared pointer in DB options. There's no reason to pass again shared pointer among internal functions. Raw pointer is sufficient and efficient.

Test Plan: make check

Reviewers: dhruba, MarkCallaghan, igor

Reviewed By: dhruba

CC: leveldb, reconnect.grayhat

Differential Revision: https://reviews.facebook.net/D14289
Create new log file outside the dbmutex.

Summary:
All filesystem Io should be done outside the dbmutex. There was one place
when we have to roll the transaction log that we were creating the new log file
while holding the dbmutex.

I rearranged this code so that the act of creating the new transaction log
file is done without holding the dbmutex.  I also allocate the new memtable
outside the dbmutex, this is important because creating the memtable
could be heavyweight.

Test Plan: make check and dbstress

Reviewers: haobo, igor

Reviewed By: haobo

CC: leveldb, reconnect.grayhat

Differential Revision: https://reviews.facebook.net/D14283
[RocksDB] Interface changes required for BackupableDB

Summary: This is part of https://reviews.facebook.net/D14295 -- smaller diff that is easier to review

Test Plan: make asan_check

Reviewers: dhruba, haobo, emayanke

Reviewed By: emayanke

CC: leveldb, kailiu, reconnect.grayhat

Differential Revision: https://reviews.facebook.net/D14301