remove start/pauseNetwork

This implementation doesn't scale.  Could be replaced with a single TVar that
every writer checks, but I'm not sure I'll ever want it anyway.
WIP: maybe optimize some input handling
clean up warnings, add some types
fix bug in dgChainHeads handling

left-biased <> meant that new updates weren't being written.

We also don't need the ChainCache, since ChainHeads will contain
every known label
minor cleanups
clean up STM.Fence, add to .cabal
Impulse.Core: clean up warnings
start to clean up DynMod code
modify runUpdates to take a lock properly.