POSIX `send` is thread safe, to take advantage of this the code has been
changed to construct a contiguous buffer and perform the send in a
single operation preventing any risk of a race condition.
Only the main channel still requires an interlock as the VD agent
requires multiple sends to transmit a full buffer.
Returning -1 from a function with bool as return argument is the same as
returning true. If the channel is not connected, return false instead to
indicate the error.
