Addressed various initialization issues and patterns

[thehenrytsai]

There are more to be done, but this is a good start.

• Fixed confusing patterns of existing async method take callbacks, and yet return immediately without invoking and awaiting on the callback.
• Introduced DwnServer.stop() for clarity and consistency.
• Introduced HttpApi.close() for clarity and consistency.
• Added missing removeProcessHandlers as counterpart to setProcessHandlers for proper clean up.
• Added didResolver to DwnServerOptions for overriding DID Resolver cache LevelDB implementation in tests to prevent DB locking issues.
• Used Poller to make some tests more stable.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 96.87500% with 5 lines in your changes missing coverage. Please review.

Project coverage is 95.64%. Comparing base (704cbb4) to head (6470a54).

Files	Patch %	Lines
src/dwn-server.ts	96.15%	3 Missing :warning:
src/http-api.ts	93.10%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #143 +/- ## ========================================== + Coverage 94.77% 95.64% +0.86% ========================================== Files 27 27 Lines 2756 2869 +113 Branches 353 374 +21 ========================================== + Hits 2612 2744 +132 + Misses 143 124 -19 Partials 1 1 ```

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