Enforce execution order using mutexes

[jupenur]

Fixes #223:

• The update listener is no longer responsible for clearing caches; caches are cleared in WebcatRequestHandler.bind, where a mutex ensures execution does not interleave with #onRequest.
• A mutex in NamespacedKVStore operations ensures they cannot interleave with operations of the same instance. No guarantees are provided across instances.
• WebcatDatabase.updateList no longer calls enrollments.clear
• Multiple EnrollmentUpdater.retryIfFailed calls only call update once if the update succeeds.

[jupenur]

This is probably easiest to review one commit at a time. Each commit pretty much fixes one issue. I could've opened separate PRs for each one but this way there's less to merge.

[lsd-cat]

Hey, sorry for the delay here! I didn't have the continuous time needed to followup and understand, but will ensure do so by this week.