MINOR: channel: add channel_in_transit()

This function returns the amount of bytes in transit in a channel's buffer,
which is the amount of outgoing data plus the amount of incoming data bound
to the forward limit.

(cherry picked from commit 1a4484dec8488cdd14c52e6cb93198e3c8f83942)
[wt: this is not a bug but the next bugfix needs it]
1 file changed