[Mempool] Trying to allocate from a full arena

[mratsim]

https://travis-ci.com/mratsim/weave/jobs/271436702#L414-L426

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Running [] weave/memory/memory_pools.nim

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Single-threaded: System alloc for 100 blocks: 0.7871 s

Single-threaded: Pool   alloc for 100 blocks: 0.4178 s

Multi-threaded: System alloc: 0.0178 s

fatal.nim(39)            sysFatal

Error: unhandled exception: contracts.nim(86, 15) `

arena.meta.used < arena.blocks.len` 

    Contract violated for pre-condition at memory_pools.nim:359

        arena.meta.used < arena.blocks.len

    The following values are contrary to expectations:

        62 < 62  [Worker N/A]

 [AssertionError]

weave/weave/memory/memory_pools.nim

Lines 356 to 367 in 5a5f2cb

	func allocBlock(arena: var Arena): ptr MemBlock {.inline.} =
	## Allocate from an arena
	preCondition: not arena.meta.free.isNil
	preCondition: arena.meta.used < arena.blocks.len
	arena.meta.used += 1
	result = arena.meta.free
	unpoisonMemRegion(result, WV_MemBlockSize)
	# The following acts as prefetching for the block that we are returning as well
	arena.meta.free = cast[ptr MemBlock](result.next.load(moRelaxed))
	postCondition: arena.meta.used in 0 .. arena.blocks.len