Use buildbox-casd as a CAS proxy in e2e test
Start testing the behavior of buildbox-casd in the e2e test. This spins up a buildbox-casd
instance to talk to buildgrid
and communicates with buildbox-worker through a unix socket. Also bumped up the log levels to provide lots of output to troubleshoot if things don't work as expected.
Closes #3 (closed)