only flush if write was successful; if it throws when writing, make sure that items in a transaction are aborted

First pass, and we almost certainly want to rip this out, but getting some sanity numbers on Ubuntu before going further.

make sure that _writeTickCount is set *before* it gets in the queue (otherwise heartbeat will freak)

* Build: let's try and get Ubuntu working
* Eliminate LibraryTargetFrameworks
* Tests: only build net462 on Windows
* Oh Linux, you're gonna be fun
* More line ending love
* Move perf counter and thread bits to PerfCounterHelper

track the current message so we can give a good error in the case of fail; prefer Volatile.Read over CEX /null/null

It forces GC which can cause a pause...move it out of the normal test run.

don't be so aggressive about adding PINGs if we're already backed up; no need to report QUIT as faulted if the socket died (that's always a race)

make sure that as many places as possible see PhysicalConnection's state (via ToString(), rather than physicalName)

ensure that we actually update the last-write-time so we don't constantly heartbeat; pre-emptively set last-write-time *before* sending heartbeat, to prevent over-runs; track physical's write-state