Get creation of flat buffer off of critical async path

[TylerADavis]

Currently the creation of a flat buffer takes a non trivial amount of time, slowing down asynchronous operations. It may be beneficial to move the burden for creating flat buffers onto one of the worker threads.

A few approaches:

1. Reuse flatbuffers for write operations, this is done in Speed improvements #90
2. Have sender thread responsible for creating flat buffers
3. Create a fourth thread that solely creates flat buffers, and then places them in send queue (seems like a bad idea)

@jcarreira let me know what you think. Additionally, were all flat buffers taking a significant amount of time to create, or just the flat buffers for write operations?