Add SSL tests; Fix SSL bugs; Port over TCP improvements from Wallaroo

[dipinhora]

This PR includes a number of TCP and SSL fixes/improvements (if desired, I can break them apart into separate PRs):

• Cleanup TCPConnection GC-safety mechanism for writev buffers
  Prior to this commit, the TCPConnection pending_writev buffer had to
  have a secondary buffer called _pending to ensure that the data
  wouldn't get GC'd too early by the runtime.

  This commit removes that hack and replaces it with a slightly different
  hack based on the strategy used in File in PR Fix GC-safety issue with writev pointers in File. #2775. The hacky part
  is that now we have two buffers for writev data. One for windows
  (_pending_windows) and one for non-windows (_pending_posix) to
  account for the difference in order of the struct between windows
  and posix. This, however, seems less bad than the previous hack of
  having the secondary buffer to ensure GC safety.

  resolves Pointer._offset is private, but Array.cpointer takes an offset argument. #2782
  resolves Clean up TCPConnection GC-safety mechanism for writev buffers. #2779

• Make reading via TCPConnection re-entrant safe
  Port over bc6f78bcaad55156393753c540befe9f581065bc from wallaroo.

  There's a nasty possible bug in TCPConnection before
  applying this change.

  Because _mute and _unmute are synchronous methods on
  TCPConnection and because they can be called inside
  _pending_reads via code executing in notify.received, its
  possible to mute and unmute a source while executing
  _pending_reads.

  Prior to this change, if you were to unmute from inside a pending
  reads via the above scenario, _unmute would synchronously call
  _pending_reads to re-enter that function and start processing
  data out of order. Further, we could end up in an infinite loop
  so long as enough data existed.

• Change how TCP connection reads data to improve performance
  Port over of the "faster tcp" logic from wallaroo PRs:

  Update TCP Source and Data Channel WallarooLabs/wally#2203
  Update _pending_reads TCP logic WallarooLabs/wally#2563

  That provides a significant performance improvement to the
  throughputs and latencies for the wallaro performance test.

• Make TCPConnection yield on writes to not hog cpu
  Port over of the following PR from wallaroo:

  Add _write_again behavior to TCP actors with _pending_writes WallarooLabs/wally#2565

  This commit adds a _write_again to the TCPConnection actor
  and modifies the _pending_writes function to yield by calling
  _write_again after having sent at least _max_size bytes.
  This will allow other actors and GC to run when a large amount
  of data has been queued up to be sent.

• Add SSL tests and fix some SSL related bugs
  This commit adds SSL tests for expect, writev, mute, unmute, and
  throttle. As part of adding these tests, some bugs were encountered
  and fixed to get the tests to pass.

[SeanTAllen]

@dipinhora can you break apart into separate PRs and add CHANEGLOG entries for those to the PRs after opening them?

[SeanTAllen]

this is a "bug" i just fixed in lori. see ponylang/lori@c61b165

[dipinhora]

Yeah... I noticed that "bug" but was trying not to do more than a "port" of the code as part of this.

[SeanTAllen]

this change is non-windows only, correct?

[dipinhora]

Correct, there is no logical change to the windows code. The only change to the windows code is to migrate from _read_len to _read_buf_offset.

[dipinhora]

@SeanTAllen I've opened (adding changelog entries soon):

#3174
#3175
#3176
#3177

The PR for the faster tcp performance related change will have to wait for the TCP re-entrant change to be merged first.

[dipinhora]

@SeanTAllen changelog entries added. But it seems that my adding [skip ci] to the changelog commits might not be ideal because it prevents the travis/circleci/appveyor tests from running.

[SeanTAllen]

Yeah, you'd need to try and rewrite that commit to not have [skip ci]

[dipinhora]

yay! let me change that and force push.