Fix a group commit bug in LogAndApply

Summary:
EncodeTo(&record) does not overwrite, it appends to it.

This means that group commit log and apply will look something like:
record1
record1record2
record1record2record3

I'm surprised this didn't show up in production, but I think the reason is that MANIFEST group commit almost never happens.

This bug turned up in column family work, where opening a database failed with "adding a same column family twice".

Test Plan: Tested the change in column family branch and observed that the problem is gone (with db_stress)

Reviewers: dhruba, haobo

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16461