ResourceManager trait impl

Introduces the DefaultResourceManager
struct. The core of methods that will
be used to inform the HTLC forward
decisions are add/resolve_htlc.

- add_htlc: Based on resource
availability and reputation, it
evaluates whehther to forward or
fail the HTLC.

- resolve_htlc: Releases the
bucket resources used from a
HTLC previously added and updates
the channel's reputation based
on HTLC fees and resolution times.

Author: elnosh