feat(dapi): serve even if tenderdash is not connected

[shumkov]

Issue being fixed or feature implemented

During activation or any other reason when Tenderdash RPC is not responding we still want DAPI to respond to RPC requests which aren't required Tenderdash to operate.

What was done?

• Do not block API to start if tenderdash is not connected
• Respond with Unavilable gRPC error from endpoints that require Tenderdash to be responding
• Removed two unused endpoints

How Has This Been Tested?

Tested locally

Breaking Changes

None

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated relevant unit/integration/functional/e2e tests
• I have added "!" to the title and described breaking changes in the corresponding section if my code contains any
• I have made corresponding changes to the documentation if needed

For repository code-owners and collaborators only

• I have assigned this pull request to a milestone

[pshenmic]

utACK