Fix race in TestWithNetworkConnections

There was a nasty bug where a connection-failed interrupt was
run afer any interruptible calls were made, making the next
test-case fail due to interruption.
Support registration of arbitrary NetworkParameters.
Add Block.removeTransaction() (TODO: TESTME)
Allow NetworkParameters to specify the block count for retarget.
Allow NetworkParameters to override the PoW function
Litecoin: Make bitcoin networkparameters constants protected
Litecoin: Make SeedPeers non-public (its Bitcoin-specific)
Litecoin: LitecoinNetworkParameters