Add watchtower feature to watch through multiple URLs

[Frixoe]

Problem

Watchtower could only use one URL so during outages it would seem as if only that RPC might have been affected.

Summary of Changes

Added feature to pass multiple URLs to the CLI tool

• Latency and health of every URL is evaluated and recorded at the start of the program and after every 10 iterations of the main loop.
• Only the lowest latency URL is picked for operations
• Latency for current URL is measured on every iteration
• At the end of every iteration, the current URL's latency is compared to all existing URLs latencies since last evaluation. If any URL is found to have a lower latency, the first one in the evaluation loop is picked to be the current URL.

Fixes #

[steviez]

@Frixoe - Thanks for the contribution. We have recently forked this repo, and further work will be done in:
https://github.com/anza-xyz/agave

Feel free to re-open a new PR there; the PR against this repo will not be reviewed / merged.

[Frixoe]

@Frixoe - Thanks for the contribution. We have recently forked this repo, and further work will be done in: https://github.com/anza-xyz/agave

Feel free to re-open a new PR there; the PR against this repo will not be reviewed / merged.

Got it. I've opened a pull request on the new repo too. Please take a look.

anza-xyz#161