testbed/reader-writer: test file locks

Update the reader/writer test to use file locks for actual transactional
updates.

Test Plan: $ ./testbed/reader-write/run.sh
phlsys_fs: add context managers for file locks

Wrap file opening and locking up in context managers.

Update the reader-writer test to exercise the context managers.

Test Plan:
$ ./precommit.sh
$ ./testbed/reader-writer/run.sh
WIP