gltrace: transport buffering and context management

This patch adds two improvements:
1. Protobuf messages are buffered and sent in chunks.
2. Multiple EGL contexts are handled properly: Corresponding
to each EGLContext, a GLTraceContext with a unique ID is created.
On eglMakeCurrent, the appropriate GLTraceContext is set and is
used while tracing subsequent GL Calls in that thread.

Change-Id: I34076376d3e5af205c87c7396ea47659844abd6e