Add ability to pause build-slaves.

Signed-off-by: Tom Prince <tom.prince@ualberta.net>
Start implementing option to pause a slave.

Signed-off-by: Tom Prince <tom.prince@ualberta.net>
- New WIP for Clean shutdown.
Raise ImportError instead of exiting, if buildbot.test can't be imported.

This apparently causes issue with 'pydoc -k', and is in general not nice.
Trial still fails with a sensible error message.

This is a slave-side reflection of commit 126e02341c76bd5a027c2ca5a8467aa5708228f2
don't run sqlalchemy betas, either
Don't claim to obfuscate passwords with master-side source step.

We don't currently have any way of obfuscating arbitrary things sent from the
master, so don't claim that we can.
Fixed try builds not being scheduled when more than one is pending
Merge branch 'buildbot-0.8.7'
run the master for a split second in integration tests
Merge remote-tracking branch 'lorentz/master'
RemoteCommand: Fix handling of collectStderr.

This argument has never worked, since it was added, but it wouldn't
fail unless something was actually received on stderr.
Add some better debugging for buildstep test failures.
Monkey patch recent gatherResults on old versions of twisted.

Newer versions of gatherResults support consumeErrors, which we would like to use.
Merge branch 'gatherResults'
- Changed graceful to clean.  Added also some docs.
Handle slave disconnections during build start better

This:
 - checks for a slave disconnection at the last possible moment
 - handles exceptions from startBuild's deferred better
 - treats errors in startBuild as EXCEPTION, not FAILURE

Fixes #2427.
comment about maybeDeferred use; better log.err message
Reference relevant bug.
Merge pull request #612 from djmitche/bug2427

Handle slave disconnections during build start better.
Use eventual.eventually more consistently.