Refactor dispatcher timeout

[ian-shim]

Why are these changes needed?

Currently, the dispatcher DisperseBatch method takes context in the parameter but uses the fixed timeout specified in its config.
This doesn't allow the caller to use variable timeouts for different requests which is necessary for minibatcher.
This PR makes two changes:

• It removes the fixed timeout in dispatcher config, and adds a timeout parameter for explicitly setting the timeout for each call
• It makes sendChunks a public method

Checks

• I've made sure the lint is passing in this PR.
• I've made sure the tests are passing. Note that there might be a few flaky tests, in that case, please comment that they are not relevant.
• Testing Strategy
  • Unit tests
  • Integration tests
  • This PR is not tested :(