Implement light client leaf fetching and caching with fixed stake table

[jbearer]

See task

This PR does

• Implement leaf fetching and verification for the light client
• Support a local fixed size cache so we don't have to fetch the same leaves over and over again. The cache can be in memory or persisted in a file, either way we use a SQLite interface

This PR does not

• Implement stake table fetching. Of course leaf verification requires a stake table. This PR just uses a fixed stake table always. The next task will be to support dynamic stake table fetching, which will depend on leaf verification (recursively)

[Sneh1999]

Added some nits but looks good! Thanks for all the comments in the code, helps me understand

[Sneh1999]

LGTM!