soak lfu add + fix detached nodes

[bitfaster]

This test exposed a bug - there was logic missing to handle nodes being removed while being read/updated.

In this case, the node was previously in probation, so Position=Probation, and the node's list is null (since it was removed). The debug assert attempts to verify that the probation node is attached to the list, which it is not.

Error Message:
   System.InvalidOperationException : Node is already attached to a different list.
  Stack Trace:
     at BitFaster.Caching.Throw.InvalidOp(String message) in /_/BitFaster.Caching/Throw.cs:line 29
   at BitFaster.Caching.Lfu.LfuNodeList`2.ValidateNode(LfuNode`2 node) in /_/BitFaster.Caching/Lfu/LfuNodeList.cs:line 140
   at BitFaster.Caching.Lfu.LfuNodeList`2.Remove(LfuNode`2 node) in /_/BitFaster.Caching/Lfu/LfuNodeList.cs:line 61
   at BitFaster.Caching.Lfu.ConcurrentLfu`2.PromoteProbation(LfuNode`2 node) in /_/BitFaster.Caching/Lfu/ConcurrentLfu.cs:line 691
   at BitFaster.Caching.Lfu.ConcurrentLfu`2.OnWrite(LfuNode`2 node) in /_/BitFaster.Caching/Lfu/ConcurrentLfu.cs:line 679
   at BitFaster.Caching.Lfu.ConcurrentLfu`2.Maintenance(LfuNode`2 droppedWrite) in /_/BitFaster.Caching/Lfu/ConcurrentLfu.cs:line 588
   at BitFaster.Caching.Lfu.ConcurrentLfu`2.AfterWrite(LfuNode`2 node) in /_/BitFaster.Caching/Lfu/ConcurrentLfu.cs:line 470
   at BitFaster.Caching.Lfu.ConcurrentLfu`2.TryAdd(K key, V value) in /_/BitFaster.Caching/Lfu/ConcurrentLfu.cs:line 205
   at BitFaster.Caching.Lfu.ConcurrentLfu`2.GetOrAdd(K key, Func`2 valueFactory) in /_/BitFaster.Caching/Lfu/ConcurrentLfu.cs:line 224
   at BitFaster.Caching.UnitTests.Lfu.ConcurrentLfuSoakTests.<>c__DisplayClass7_0.<WhenConcurrentGetAndUpdateCacheEndsInConsistentState>b__0() in D:\a\BitFaster.Caching\BitFaster.Caching\BitFaster.Caching.UnitTests\Lfu\ConcurrentLfuSoakTests.cs:line 125

[coveralls]

coverage: 98.582% (+0.2%) from 98.344% when pulling dfd03ce on users/alexpeck/soaklfuadd into 60e78bf on main.