Distinguish maximum HTLC from effective capacity

Distinguish maximum HTLC from effective capacity

When determining a route, find_route used the smaller of a channel's
maximum HTLC and capacity for capping how much could be sent through a
channel across all paths. However, the maximum HTLC limit should be
applied per path and be limited by the effective capacity across all
paths.

Define an AvailableLiquidity abstraction for handling this behavior
correctly. Change EffectiveCapacity to prefer the channel's capacity
over its maximum HTLC limit. For both cases, the previous behavior has
not changed when the channel's capacity is unknown.