Add input/output buffering to ThriftEventSource/Sink

Some simple buffering should improve network performance quite a bit. I've so far implemented this only on the sending side (ThriftEventSink) and performance seems to have improved nicely, though I don't have good numbers for this yet. There are two possible solutions:

1) Use the native Thrift TFastFramedTransport (buffered version of TFramedTransport); lazy socket changes will need to be ported over (for TSaneServerSocket)
2) Wrap the existing code with a buffered version (see TBufferedTransport in patch)

I used the latter method for a simple test implementation only on the sending (sink) side.