This commit implements `PtMgr`, a pluggable transport manager
responsible for keeping track of spawned PTs and spawning them to
satisfy client requests on demand.
It does this in two parts: the `PtMgr` type exported to the rest of the
code, and the background `PtReactor` that actually does the spawning;
this design ensures that only one attempt to spawn a PT is active at a
time, and will prove useful later for implementing e.g. timeouts.
A few changes were necessary to the rest of the code in order to make
this all work out. Namely:
- `TransportRegistry`'s API didn't make any sense for two reasons:
- It wasn't feasible for implementors to implement `ChannelFactory`,
since that'd require constructing a `ChanBuilder` (which requires a
bootstrap reporting event sender).
- Treating the PT manager as a registry is over-general; it's only
necessary for it to spawn pluggable transports, so saddling it with
other concerns didn't make any sense.
- (It's possible to get extensibility for arbitrary user customization
by just letting the user swap in a new `ChannelFactory`, anyway.)
- Therefore, the `PtMgr` implements the new `AbstractPtMgr` trait, which
is far more narrowly focused; this only exists to solve a dependency
loop, and is documented as such.
- This provides a `TransportHelper` instead of a `ChannelFactory`.
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