Refactor TLCs related fields in channel state

[contrun]

Currently the following two issues #30 #31 require us to save TLCs with more information to the channel state. It seems that it would be better if we can save TLC to a single field tlcs instead of pending_offering_tlcs, pending_received_tlcs and to_be_committed_tlcs. But we need also need to save the commitment number when this TLC is added and the commitment number this TLC is removed.

This issue potentially depends on #43 and #44 .

[contrun]

Done in #72.