[RocksDB] Expose count for WriteBatch

Summary: As title. Exposed a Count function that returns the number of updates in a batch. Could be handy for replication sequence number check.

Test Plan: make check;

Reviewers: emayanke, sheki, dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D11523

db/write_batch.cc

@@ -45,6 +45,10 @@ void WriteBatch::Clear() {
   rep_.resize(kHeader);
 }
 
+int WriteBatch::Count() const {
+  return WriteBatchInternal::Count(this);
+}
+
 Status WriteBatch::Iterate(Handler* handler) const {
   Slice input(rep_);
   if (input.size() < kHeader) {