The LSP code had numerous places where competing threads could take out
out locks in different orders, making it very prone to deadlocks.
This commit sidesteps the entire issue by switching to a single lock.
The above is a little white lie: the Sources struct still uses a mutex
internally to avoid having to boil the ocean (because being honest about
what it does involves changing all its methods to `&mut self` but that
ripples out extensively...) I'll save that one for another day.
