Only allow a single pending channel per-peer

[Roasbeef]

Currently the fundingManager, and the rpcServer will happily allow either a caller or remote peer to create multiple pending channels at a time. This behavior should instead be restricted to only allow a single pending channel at a time per-peer. Such a constraint acts as a defense against a slow-loris like DoS attack wherein a peer creates hundreds of thousands of pending channels, never intending to complete the funding workflow for them.

Steps to completion:

• the fundingMgr should reject all requests to either process or initialize a new channel funding workflow if one already exists for the targeted peer.
• the lnwire.ErrorGeneric messagesshould be sent to the offending peer if the constraint is violated

[andrewshvv]

I will try to do this one.

[Roasbeef]

Fixed by #36.

Closing.