Major refactor of net.go, significant parts of
client, and a few other small changes, in an
attempt to seperate out network code, simplify msg
flows and reduce the chance (and need for mitagations
for) deadlocks. Aims to be non-breaking from users
perspective.

This is an initial pull request for review; it passes
the existing tests but will need more testing. Submitting
it to see if there is an appetite for this level of change
and agreement on whether the changes add value.

closes 372

Signed-off-by: Matt Brittan <matt@brittan.nz>