lnpeer: increase max_accepted_htlcs (5->30)

Counter-intuitively, the motivation is to be able to *send* more htlcs,
for MPP trickery. We don't offer more htlcs than this limit, to be
conservative with what we send, and to interoperate with clightning.

defaults of other clients:
eclair: 30
clightning: 30
lnd: 483

electrum/lnchannel.py

@@ -781,6 +781,7 @@ class Channel(AbstractChannel):
         if len(self.hm.htlcs_by_direction(htlc_receiver, direction=RECEIVED, ctn=ctn)) + 1 > chan_config.max_accepted_htlcs:
             raise PaymentFailure('Too many HTLCs already in channel')
         # however, c-lightning is a lot stricter, so extra checks:
+        # https://github.com/ElementsProject/lightning/blob/4dcd4ca1556b13b6964a10040ba1d5ef82de4788/channeld/full_channel.c#L581
         if strict:
             max_concurrent_htlcs = min(self.config[htlc_proposer].max_accepted_htlcs,
                                        self.config[htlc_receiver].max_accepted_htlcs)