core: add finalizer checks for ManagedChannels (#3452)

* core: add finalizer checks for ManagedChannels

Cleaning up channels is something users should do.  To promote this
behavior, add a log message to indicate that the channel has not
been properly cleaned.

This change users WeakReferences to avoid keeping the channel
alive and retaining too much memory.  Only the id and the target
are kept.  Additionally, the lost references are only checked at
JVM shutdown and on new channel creation.  This is done to avoid
Object finalizers.

The test added checks to see that the message is logged.  Since
java does not allow forcing of a GC cycle, this code is best
effort, giving up after about a second.  A custom log filter is
added to hook the log messages and check to see if the correct
one is present.  Handlers are not used because they are
hierarchical, and would be annoying to restore their state after
the test.

The other tests in the file contribute a lot of bad channels.  This
is reasonable, because they aren't real channels.  However, it does
mean that less than half of them are being cleaned up properly.
After trying to fix a few, it is too hard to do.  It would only
serve to massively complicate the tests.

Instead, this code just keeps track of how many it wasn't able to
clean up, and ignores them for the test.  They are still logged,
because really they should be closed.