Circular dependency between iocore and tscore

[jpeach]

If you try to run tests without doing a full build first, the build fails because of ordering issues:

$ make -j4 check
...
make[3]: Entering directory '/home/jpeach/src/trafficserver/src/tscore'
...
make[3]: *** No rule to make target '../../iocore/eventsystem/libinkevent.a', needed by 'test_tscore'.  Stop.

However, it's not possible to re-order the build so that this works, because iocore and tscore depend on each other.

In tscore, the MTHashTable class uses ProxyMutex from iocore for locking. ProxyMutex pulls in EThread and all sorts of other iocore symbols. iocore depends on tscore all over the place, which seems reasonable and intended.

AFAICT, MTHashTable is only used in one place (QUIC support in iocore, see #2478).

[jpeach]

Looks like a reasonable solution could be to move MTHashTable into iocore as a private implementation detail.

[jpeach]

Since internal QUIC is a red herring (see #9670), we can just remove MTHashTable again.

[jpeach]

Since internal QUIC is a red herring (see #9670), we can just remove MTHashTable again.

Ugh, except that some of the QUIC code is used? 🙄