Consider adding a time interval based write coalescer

Currently, the connection performs coalescing when items are being dequeued from the write queue. This write queue action can only be executed one at a time and it is triggered by the socket send callback and requesthandlers themselves. The requesthandler trigger will only have an effect if the connection is idle at that time.

The java driver has a write coalescer that runs on a timer. We should consider benchmarking this approach on the C# driver.