tests/blacklists/fsmonitor

wireproto: support for receiving multiple requests

Now that we have request IDs on each frame and a specification
that allows multiple requests to be issued simultaneously,
possibly interleaved, let's teach the server to deal with that.

Instead of tracking the state for *the* active command request,
we instead track the state of each receiving command by its
request ID. The multiple states in our state machine for processing
each command's state has been collapsed into a single state for
"receiving commands."

Tests have been added so our branch coverage covers all meaningful
branches.

However, we did lose some logical coverage. The implementation
of this new feature opens up the door to a server having partial
command requests when end of input is reached. We will probably
want a mechanism to deal with partial requests. For now, I've
tracked that as a known issue in the class docstring. I've
also noted an abuse vector that becomes a little bit easier to
exploit with this feature.

Differential Revision: https://phab.mercurial-scm.org/D2870

# Blacklist for a full testsuite run with fsmonitor enabled.
# Used by fsmonitor-run-tests.
# The following tests all fail because they either use extensions that conflict
# with fsmonitor, use subrepositories, or don't anticipate the extra file in
# the .hg directory that fsmonitor adds.

#### mainly testing eol extension
test-eol-add.t
test-eol-clone.t
test-eol-hook.t
test-eol-patch.t
test-eol-tag.t
test-eol-update.t
test-eol.t
test-eolfilename.t

#### mainly testing largefiles extension
test-issue3084.t
test-largefiles-cache.t
test-largefiles-misc.t
test-largefiles-small-disk.t
test-largefiles-update.t
test-largefiles-wireproto.t
test-largefiles.t
test-lfconvert.t

#### mainly testing nested repositories
test-nested-repo.t
test-push-warn.t
test-subrepo-deep-nested-change.t
test-subrepo-recursion.t
test-subrepo.t

#### fixing these seems redundant, because these don't focus on
#### operations in the working directory or .hg
test-debugextensions.t
test-extension.t
test-help.t