Exclude cells used in the pending funding tx negotiation so that cocurrent funding tx will not conflict with each other.

• Add data structure to manage the exclusion list
• Apply exclusion list before building tx
• Update exclusion list after building tx
• Remove tx from exclusion list when channel has been established
• Add a test case for in-memory cells exclusion (blocked by [funding] Run tx tracing concurrently #348)
• Persist funding txs
• Restore exclusion list after restart
• Add RPC to cancel a pending funding tx