fix atomic factory soak test

[bitfaster]

Soak test for AtomicFactory fails intermittently:

[xUnit.net 00:00:59.66]     AtomicFactorySoakTests.WhenGetOrAddIsConcurrentValuesCreatedAtomically [FAIL]
  Failed Atomic.AtomicFactorySoakTests.WhenGetOrAddIsConcurrentValuesCreatedAtomically [26 ms]
  Error Message:
   Expected counters.Sum(x => x) to be 1024, but found 1071 (difference of 47).

This is due to items being evicted by ConcurrentLru (which is not expected given that item count falls within capacity). Switch to a pre-allocated ConcurrentDictionary, and investigate the warmup bug separately.

Also moved the volatile read in the AtomicFactory code.

[coveralls]

coverage: 97.744% (-0.05%) from 97.791% when pulling 40a71ee on users/alexpeck/atomicvol into 42f1f11 on main.