Introduce interactive signing state flags

This intoduces the INTERACTIVE_SIGNING, THEIR_TX_SIGNATURES_SENT, and
OUR_TX_SIGNATURES_SENT funded state flags.

A top-level state flag for INTERACTIVE_SIGNING was avoided so that this
work is compatible with splicing as well as V2 channel establishment
(dual-funding).

This commit also ensures that `ChannelPending` is only emitted after
peers exchange `tx_signatures`.

Author: dunxen