[keyserver] Introduce getOLMUtility(...) and initialize OLM on keyserver start

Summary: Initialize OLM by calling await olm.init() on keyserver start and introduce getOLMUtility to potentially access cached OLMUtility... or instantiate a new one.

Test Plan:

• Logged in a bunch of times and ensured things worked as expected.
• Set breakpoint on cache hit:

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6901