mutexAllocator and netVCAllocator in-use count continually growing

Running HTTP traffic through 3.2.0 we see the in-use count from the mutexAllocator and netVCAllocators continually growing.

An example allocator dump before a test:
88064 | 2064 | 688 | memory/netVCAllocator
1566720 | 1564800 | 80 | memory/mutexAllocator

After the test has completed (about 15 minutes of traffic):

13914112 | 629520 | 688 | memory/netVCAllocator
2570240 | 2376480 | 80 | memory/mutexAllocator

We never see the in-use counters fall back to the value set before the test. If we start the test again then we get further growth. Running for longer or under heavy load the growth is much clearer:

6791976960 | 6791970080 | 80 | memory/mutexAllocator