Motivation:
`useCacheForAllThreads` may be false which disables memory caching
on non netty threads. Setting this argument or the system property
makes it impossible to use `PooledByteBufAllocator`.
Modifications:
Delayed the check of `freeSweepAllocationThreshold` in
`PoolThreadCache` to after it knows there will be any caches in
use. Additionally, check if the caches will have any data in them
(rather than allocating a 0-length array).
A test case is also added that fails without this change.
Results:
Fixes#7194
d2cb51bc2e