Enable users to choose which dispatchers to use for Client and Document

[skhugh]

What this PR does / why we need it?

Enabled users to choose which dispatcher to use for Client and Document, rather than using a fixed single-threaded dispatcher.

Any background context you want to provide?

While applying Yorkie to a text editor it was found that using a background thread for document updates could cause problems as users can input text while remote changes for document are being applied.
It is very hard to control Android's text input as it is mostly controlled by Android framework, therefore using a main thread for document updates should be recommended.
But for usecases where developer can control the sequence of data changes in detail, using a background thread can still be advantageous.

I also changed Client and Doucment to Closeable as default single-thread dispatcher should be closed.

What are the relevant tickets?

Fixes #

Checklist

• Added relevant tests or not required
• Didn't break anything

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (47b2323) 79.62% compared to head (6b6f0a4) 79.62%.

Files	Patch %	Lines
yorkie/src/main/kotlin/dev/yorkie/core/Client.kt	85.71%	0 Missing and 1 partial ⚠️
...ie/src/main/kotlin/dev/yorkie/document/Document.kt	85.71%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #153      +/-   ##
==========================================
- Coverage   79.62%   79.62%   -0.01%     
==========================================
  Files          63       63              
  Lines        3843     3852       +9     
  Branches      601      603       +2     
==========================================
+ Hits         3060     3067       +7     
  Misses        459      459              
- Partials      324      326       +2     

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