fix: improved locking of assembler state

[loshz]

Which problem is this PR solving?

• Load test HTTP request count does not come close to events received by Honeycomb #209

Short description of the changes

During the debugging of suspected load testing issues, the following bugs were found and fixed:

• Switch from sync.Map to regular map w/ mutex 3b8a008
• Ensure stats accessed concurrently are explicitly atomic fe2ebd3
• Remove unused sync.Waitgroup from TCP stream factory 2e039ad

How to verify that this has the expected result

On my local Linux machine, I compiled the main branch and the changes in this PR, and ran them concurrently while sending several thousand HTTP requests to a random netcat server. The results are identical:

Main:

DBG TCP assembler stats IPdefrag=0 active_streams=0 conn_rejected_FSM=0 event_queue_length=500 goroutines=6 rejected_FSM=0 rejected_Options=0 source_dropped=0 source_if_dropped=0 source_received=4000 total_TCP_bytes=97500 total_streams=500 uptime_ms=25450
6:33PM DBG Stopping TCP assembler assembler_page_usage="pageCache: used: 0:" closed=500

PR:

DBG TCP assembler stats IPdefrag=0 active_streams=0 conn_rejected_FSM=0 event_queue_length=500 goroutines=6 rejected_FSM=0 rejected_Options=0 source_dropped=0 source_if_dropped=0 source_received=4000 total_TCP_bytes=97500 total_streams=500 uptime_ms=20920
6:33PM DBG Stopping TCP assembler assembler_page_usage="pageCache: used: 0:" closed=500

[robbkidd]

I've got a couple commits to propose tack onto these improvements:

• reuse the return vars within the function (b7762e2)
• include recording the packet count when we have a match (5b4cec8)
  • I think we missed adding that to both places prior to your updates in this PR. 🤭

[robbkidd]

I gave you bad code, @loshz. 😬