Fix the failure of stringappend_test caused by PartialMergeMulti.

Summary:
Fix a bug that PartialMergeMulti will try to merge the first operand
with an empty slice.

Test Plan: run stringappend_test and merge_test.

Reviewers: haobo, igor

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17157
[rocksdb] make init prefix more robust

Summary:
Currently if client uses kNULLString as the prefix, it will confuse
compaction filter v2. This diff added a bool to indicate if the prefix
has been intialized. I also added a unit test to cover this case and
make sure the new code path is hit.

Test Plan: db_test

Reviewers: igor, haobo

Reviewed By: igor

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17151
fallocate_with_keep_size is false for LogWrites
Add assert to MergeOperator::PartialMergeMulti to check # of operands.

Summary:
Add assert(operands_list.size() >= 2) in MergeOperator::PartialMergeMulti
to ensure it's only be called when we have at least two merge operands.

Test Plan: run merge_test and stringappend_test.

Reviewers: haobo, igor

Reviewed By: igor

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17169
Merge branch 'master' into columnfamilies