Cache fields instead of storing a ChannelKeys in ChannelMonitor

Cache fields instead of storing a ChannelKeys in ChannelMonitor

We only actually use two of the fields in ChannelKeys inside a
ChannelMonitor - the holder revocation_basepoint and the
derivation parameters. Both are relatively small, so there isn't
a lot of reason to hold a full copy of the ChannelKeys (with most
of the interaction with it being inside the OnchainTxHandler).

Further, this will avoid calling read on a `ChannelKeys` twice,
which is a somewhat strange API quirk.