[RocksDB][Bug] Look at all the files, not just the first file in TransactionLogIter as BatchWrites can leave it in Limbo

Summary:
Transaction Log Iterator did not move to the next file in the series if there was a write batch at the end of the currentFile.
The solution is if the last seq no. of the current file is < RequestedSeqNo. Assume the first seqNo. of the next file has to satisfy the request.

Also major refactoring around the code. Moved opening the logreader to a seperate function, got rid of goto.

Test Plan: added a unit test for it.

Reviewers: dhruba, heyongqiang

Reviewed By: heyongqiang

CC: leveldb, emayanke

Differential Revision: https://reviews.facebook.net/D10029