Fixed 3 Replication tests.

Fixed TestAllLeafRevisionsArePushed, TestHeaders, and TestPusherDeletedDoc.

Some key exceptions were getting swallowed,
which prevented TestHeaders from firing
the Changes callback.

Removed the early instantiation of the shared
instance of Manager, which is only used in
the Android simple demo.

Also removed some opportunities for the TaskScheduler to deadlock.
Fixed additional tests.

Allowed broader use of inlining in the TaskScheduler,
and resolved a dispose issue in the MockHttpClientFactory
that lead to a deadlock.

Fixes TestRemoteConflictResolution and TestPuller.