Limit the BlockingCollection in our socket to 1MB of data,

This fixes a bug, where the sender could send endless data and risk an
OutOfMemoryException if the receiver couldn't keep up with the
processing.
This basically re-enables flow control of the TCP protocol

1MB is an arbitrary value that I choose, because the maximum capacity of
a BlockingCollection cannot be changed later
Add a comment for the default value
Merge pull request #15 from flagbug/fix-flow-control

Limit the BlockingCollection in ReactiveSocket to 1MB of data