discovery: add comprehensive for async timestamp range queue

In this commit, we add thorough unit tests for the new asynchronous
timestamp range queue functionality. The tests verify several critical
properties and invariants of the system.

The basic functionality tests ensure that messages can be queued and
are eventually processed by calling ApplyGossipFilter. We verify that
the queue properly rejects new messages when full, returning false as
designed to prevent blocking.

Concurrent access is tested extensively to ensure the queue remains
thread-safe under heavy load. Multiple goroutines attempt to queue
messages simultaneously, and we verify that the system handles this
gracefully without data races or corruption.

The test suite includes property-based tests using the rapid framework
to verify key invariants: the queue never exceeds its capacity, all
successfully queued messages are eventually processed if the syncer
remains active, and messages maintain FIFO ordering throughout the
pipeline.

Error handling tests ensure that failures in ApplyGossipFilter don't
block the queue processor. When errors occur, the system logs them
and continues processing remaining messages, maintaining overall
system availability.

Finally, shutdown tests verify that the queue processor exits cleanly
when the syncer stops, preventing goroutine leaks and ensuring proper
resource cleanup.

Author: Roasbeef