drpcstats: add stats for bytes read/written by rpc

Change-Id: I449a3447d9b320aef0d1296e46d083ffb22130b4

drpcstream: SendCancel is busy when writing at all

SendCancel would return that it was not busy if the
stream was terminated, but there are ways for the
stream to be terminated while it is still writing.

Instead, check if any writes are happening before
the check for termination.

Change-Id: Ib3424d351f66741961cb7fe32795c0c0dfa99db2

drpcwire: limit frame size based on packet size

if packets are huge, then frames should be allowed to be too.
similarly, if packets are small, then there is no reason for
huge frames.

Change-Id: I36b46abefbd7c49bfa807c62e00c739015fc3703

drpcwire: expose buffer size on Reader (#30)

The maximum was hardcoded at 4MB. This allows it to
be configurable for larger payloads. An option is
exposed in `dprcmanager.Options`.

drpcstream: return ClosedError for remote closes

In logging it's helpful to distinguish between an actual stream
error and a remote closing, which is to be expected.

Updates storj/storj#4609

Change-Id: I5e70413484820efb9d0d2c8792c9713de9f8658f

drpcstream: set cancel state more often

if a Close has started, and we Cancel, we should still
set the cancelled state while Close is still executing.
this is so that we don't surface the underying transport
being closed errors.

Change-Id: I2f3924f7b03c3952ab58b9952e4ae18ef1a5a88f

drpcserver: log callback and temporary error sleeps

It was possible before to spin a cpu core trying to
accept from the tcp listener when it always returned
a temporary error. Now, it will sleep for 500ms before
trying again.

Change-Id: I59d1503b4d14e9ded78d78c608ea4c416a4e10df

drpcstream: control for max buffer size

with many concurrent and potentially cached streams that
send large messages, dropping large buffers can help
with peak memory usage.

Change-Id: Ia99f30ff78307bea909fde813c2ecb6667a4b1d8

drcpserver: fix windows build

Change-Id: Ia4d2d138ece4a5b6d736e5e72405960191ac9ff1

drpcmanager: fix closing race condition

in some situations, newStream could be blocked forever
trying to send a stream into m.streams when the
manageStreams goroutine has already exited due to manager
terminating.

this changes the send into a select that can be canceled
by the manager terminating.

also, there was a race in the tests for the muxer where
rare failures could happen if the Route call was not
scheduled before the connection with the given prefix
was handled.

Change-Id: If2033d8b8e3b2c3e3a50e8a72337bc263458321e