drpc: ad-hoc issues while refactoring DRPC to bring stream multiplexing

This issue will track some ad-hoc steps or issues to lay the groundwork for DRPC multiplexing,
- [ ] https://github.com/cockroachdb/drpc/pull/39: Enable back the TestRandomized_Server and TestRandomized_Client
- [ ] https://github.com/cockroachdb/drpc/pull/42: Remove one-to-one packet-to-frame mapping introduced in this PR.
- [ ] Remove the assumption that non-message will always be 1 packet 1 frame.
- [ ] Remove time.Sleep from manager_test.go
- [ ] Add reusable buffer in PacketBuilder.
- [ ] Extract out manageReader as a separate entity
- [ ] add pool to invoke assemblers
- [ ] [rename invokeInfo](https://github.com/cockroachdb/drpc/pull/43#discussion_r3039089367) to something more meaningful
- [ ] remove [checkFinished](https://github.com/cockroachdb/drpc/pull/45#discussion_r3032110043)

Jira issue: CRDB-62057