[MERGE] various tweaks to push

[Michael Hudson]

Robert Collins wrote:
> This is a few tweaks all conflated, because the issues weren't clear.
> 
> Basically - we were churning locks a lot (known), and code depended on
> the locks being released for some methods (bad). 

Super glad to see this being worked on!

I filed some related bugs a while ago:

https://bugs.edge.launchpad.net/bzr/+bug/283457
"set_stacked_on_location doesn't take a write lock"

https://bugs.edge.launchpad.net/bzr/+bug/283479
"set_stacked_on_location leaves a locked repossitory with an unlocked
fallback repository"

I think they can both be closed when this branch lands.

Cheers,
mwh