MPSC count can become negative

[mratsim]

Even though the count is done on the consumer side which should always underestimate the real count, the number of estimated enqueued item in the MPSC channel can be negative.

See #48 (comment) and CI https://dev.azure.com/numforge/Weave/_build/results?buildId=25&view=logs

This is not blocking as this count is only informative and used for adaptative stealing and for the memory pool to trigger batch reception of memory.

This is something I actualy remarked in the past but seems to happen rarely:

weave/weave/channels/channels_mpsc_unbounded_batch.nim

Lines 208 to 220 in fffffd1

	func peek*(chan: var ChannelMpscUnboundedBatch): int32 {.inline.} =
	## Estimates the number of items pending in the channel
	## - If called by the consumer the true number might be more
	## due to producers adding items concurrently.
	## - If called by a producer the true number is undefined
	## as other producers also add items concurrently and
	## the consumer removes them concurrently.
	##
	## This is a non-locking operation.
	result = int32 chan.count.load(moAcquire)
	# For the consumer it's always positive or zero
	postCondition: result >= 0 # TODO somehow it can be -1

[mratsim]

Hopefully solved by partial model checking in #128 (memory bug on the model checker that interrupts it, ...)

[mratsim]

Still happening in the memory pool:

• https://dev.azure.com/numforge/Weave/_build/results?buildId=570&view=logs&j=3dd194dc-da28-5155-661e-b4dc81d05444&t=d13fe263-11f9-553d-3683-d0cbe4053590

========================================================================================
Running [ c  ] weave/memory/memory_pools.nim
========================================================================================
Single-threaded: System alloc for 100 blocks: 0.9514 s
Single-threaded: Pool   alloc for 100 blocks: 0.4980 s
Multi-threaded: System alloc: 0.0331 s
fatal.nim(49)            sysFatal
Error: unhandled exception: contracts.nim(86, 15) `
0 <= chan.count.load(moRelaxed)` 
    Contract violated for post-condition at channels_mpsc_unbounded_batch.nim:250
        
0 <= chan.count.load(moRelaxed)
    The following values are contrary to expectations:
        
0 <= chan.count.load(moRelaxed)  [Worker N/A]
 [AssertionError]
Error: execution of an external program failed: '/home/vsts/work/1/s/build/memory_pools '
stack trace: (most recent call last)
/tmp/nimblecache/nimscriptapi.nim(165, 16)
/home/vsts/work/1/s/weave_5847.nims(46, 8) testTask
/home/vsts/work/1/s/weave_5847.nims(32, 8) test
/home/vsts/work/1/s/NimBinaries/nim-stable/lib/system/nimscript.nim(260, 7) exec
/home/vsts/work/1/s/NimBinaries/nim-stable/lib/system/nimscript.nim(260, 7) Error: unhandled exception: FAILED: nim c  --verbosity:0 --hints:off --warnings:off --threads:on -d:release --outdir:build -r weave/memory/memory_pools.nim [OSError]
       Tip: 1 messages have been suppressed, use --verbose to show them.
     Error: Exception raised during nimble script execution