keeping a live pantsd up for reducing integration test time in travis

[cosmicexplorer]

Using pantsd in travis (and keeping it alive run-over-run) means we save probably a huge amount of time on running our integration tests, which would probably mean we save a huge amount of CI time. There is an immediate blocker to this (from @stuhood in #7320 (comment)):

All integration tests use new anonymous workdirs, intentionally, to avoid mixing of results from one test method into another. This is mostly because having to worry about state from other tests being mixed into your current test method would drive you fairly quickly insane.

I believe that this means the pidfile and other information necessary to locate a pantsd instance are also currently cleaned run-over-run. If we:

• Move the pidfile and other information out of the workdir.

We might be able to try turning on pantsd to run integration tests locally and kick the tires a bit until we can trust it enough to run in travis. Completion of this ticket is blocked by #7320.