Revision 665555

Thrift: Add writePtr(), wroteBytes() to TMemoryBuffer

Summary: This adds the discussed interface to TMemoryBuffer, as follows:

         - writePtr(size) returns a ptr you can write (size) bytes to
         - wroteBytes() lets it know you wrote that many bytes

         To do this, I refactored an:
         - ensureCanWrite(size) private func

Reviewed By: dreiss

Test Plan: works in my test environment

Revert: OK

TracCamp Project: Thrift

DiffCamp Revision: 8739