Use synchronous pipeline hops to remove windows.

[Lukasa]

Motivation:

There is an awkward timing window in the TLSEventsHandler flow where it
is possible for the NIOSSLClientHandler to fail the handshake on
handlerAdded. If this happens, the TLSEventsHandler will not be in the
pipeline, and so the handshake failure error will be lost and we'll get
a generic one instead.

This window can be resolved without performance penalty if we use the
new synchronous pipeline operations view to add the two handlers
backwards. If this is done then we can ensure that the TLSEventsHandler
is always in the pipeline before the NIOSSLClientHandler, and so there
is no risk of event loss.

While I'm here, AHC does a lot of pipeline modification. This has led to
lengthy future chains with lots of event loop hops for no particularly
good reason. I've therefore replaced all pipeline operations with their
synchronous counterparts. All but one sequence was happening on the
correct event loop, and for the one that may not I've added a fast-path
dispatch that should tolerate being on the wrong one. The result is
cleaner, more linear code that also reduces the allocations and event
loop hops.

Modifications:

• Use synchronous pipeline operations everywhere
• Change the order of adding TLSEventsHandler and NIOSSLClientHandler

Result:

Faster, safer, fewer timing windows.

[Lukasa]

@swift-server-bot test this please

[Lukasa]

CI is busted, we don't break the API here.

[Lukasa]

@swift-server-bot test this please

[Lukasa]

Flaky test, tracking in #347, re-running.

[Lukasa]

@swift-server-bot test this please

[artemredkin]

There are other places were we use pipeline modifications (in the connection pool we add/remove idle and task handlers), would they benefit from sync operations as well?

[Lukasa]

Probably, but in this instance I wanted to just tackle the ones that I could see were clearly worthwhile.