Fix LcmMetaDataCache race condition on static Flid counter

[myieye]

Somewhat similar to a previous race condition I fixed in #371.

This constructor resets a static "Flid counter", so when multiple caches are initialized in parallel our tests can stumble over duplicate Flid's in the same cache, which I think resulted in this exception:

---- SIL.LCModel.LcmInvalidFieldException : Fieldname 'Affixes' does not exist. Consider using a decorator or review the fieldname for typos.
Stack Trace:
at FwDataMiniLcmBridge.Api.FwDataMiniLcmApi.CreateEntry(Entry entry, CreateEntryOptions options) in D:\a\languageforge-lexbox\languageforge-lexbox\backend\FwLite\FwDataMiniLcmBridge\Api\FwDataMiniLcmApi.cs:line 1009
at FwLiteProjectSync.Tests.EntrySyncTestsBase.CanSyncRandomEntries(Nullable1 roundTripApiType) in D:\a\languageforge-lexbox\languageforge-lexbox\backend\FwLite\FwLiteProjectSync.Tests\EntrySyncTests.cs:line 194 --- End of stack trace from previous location --- ----- Inner Stack Trace ----- at SIL.LCModel.Infrastructure.Impl.LcmMetaDataCache.GetFieldId2(Int32 clid, String fieldName, Boolean includeBaseClasses) at SIL.LCModel.DomainServices.VirtualOrderingServices.CheckParentAndGetFieldName(ICmObject parent, Int32 flid) at SIL.LCModel.DomainServices.VirtualOrderingServices.GetOrderedValue(ICmObject parent, Int32 flid, IEnumerable1 unmodifiedSeq)
at SIL.LCModel.DomainServices.VirtualOrderingServices.GetOrderedValue[T](ICmObject parent, Int32 flid, IEnumerable1 unmodifiedSeq) at SIL.LCModel.DomainImpl.LexEntry.get_VisibleComplexFormBackRefs() at SIL.LCModel.DomainImpl.LexEntryRef.UpdateComplexFormEntryBackRefs(ICmObject thingAddedOrRemoved, Boolean fAdded) at SIL.LCModel.DomainImpl.LexEntryRef.AddObjectSideEffectsInternal(AddObjectEventArgs e) at SIL.LCModel.DomainImpl.CmObject.SIL.LCModel.ICmObjectInternal.AddObjectSideEffects(AddObjectEventArgs e) at SIL.LCModel.DomainImpl.LcmList1.AddObject(AddObjectEventArgs args)
at SIL.LCModel.DomainImpl.LcmReferenceSequence1.AddObject(AddObjectEventArgs args) at SIL.LCModel.DomainImpl.LcmList1.Add(T obj)
at SIL.LCModel.DomainImpl.LexEntry.AddComponent(ICmObject other)
at FwDataMiniLcmBridge.Api.FwDataMiniLcmApi.InsertComplexFormComponent(ILexEntry lexComplexForm, ICmObject lexComponent, BetweenPosition1 between) in D:\a\languageforge-lexbox\languageforge-lexbox\backend\FwLite\FwDataMiniLcmBridge\Api\FwDataMiniLcmApi.cs:line 1112 at FwDataMiniLcmBridge.Api.FwDataMiniLcmApi.AddComplexFormComponent(ILexEntry lexComplexForm, ComplexFormComponent component, BetweenPosition1 between) in D:\a\languageforge-lexbox\languageforge-lexbox\backend\FwLite\FwDataMiniLcmBridge\Api\FwDataMiniLcmApi.cs:line 1096
at FwDataMiniLcmBridge.Api.FwDataMiniLcmApi.<>c__DisplayClass125_0.b__0() in D:\a\languageforge-lexbox\languageforge-lexbox\backend\FwLite\FwDataMiniLcmBridge\Api\FwDataMiniLcmApi.cs:line 996
at SIL.LCModel.Infrastructure.UndoableUnitOfWorkHelper.Do(String undoText, String redoText, IActionHandler actionHandler, Action task)
...

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LCM Tests

    16 files  ±0      16 suites  ±0   2m 44s ⏱️ -12s
 2 858 tests ±0   2 838 ✅ ±0   20 💤 ±0  0 ❌ ±0 
11 380 runs  ±0  11 212 ✅ ±0  168 💤 ±0  0 ❌ ±0 

Results for commit 3a52a99. ± Comparison against base commit b87d9f9.

[myieye]

We just ran into another exception that would presumably be fixed by this PR:
sillsdev/languageforge-lexbox#2252 (comment)

[rmunn]

Like with #371, the added performance cost of locking once per constructor shouldn't be noticeable in FieldWorks where simultaneous construction pretty much never happens anyway. But it will certainly make the FieldWorks Lite tests (which create and destroy a lot of FW projects) more reliable when they're run in parallel. LGTM.