refactor(mempool): use tmsync.Waker when txs are available

[lklimek]

Issue being fixed or feature implemented

Small refactor to reuse already implemented tmsync.Waker instead of chan.

What was done?

Replaced txsAvailable chan struct{} with tmsync.Waker

How Has This Been Tested?

GHA

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 made corresponding changes to the documentation

For repository code-owners and collaborators only

• I have assigned this pull request to a milestone