Preallocate headroom for transport header in clientSendT
Preallocate headroom for transport header in clientSendT

Summary:
Currently we have 3 allocations while forming a payload:

1. Thrift message serialization
2. RpcMetadata serialization
3. Rocket transport framing header

Instead, we could preallocate headroom in 1 that would be enough for 2 and 3.

This diff add this preallocation of 128 bytes to `clientSendT`.

Reviewed By: jmswen

Differential Revision: D16587538

fbshipit-source-id: ec6d3b2dd0667b1be8fdc98b13026a40a0508049

facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift