FieldNumbers.clear() should reset lowestUnassignedFieldNumber

A call to IndexWriter.deleteAll() should completely reset the state of the index. Part of that is a call to globalFieldNumbersMap.clear(), which purges all knowledge of fields by clearing name -> number and number -> name maps. However, it does not reset lowestUnassignedFieldNumber.

If we have loop that adds some documents, calls deleteAll(), adds documents, etc. lowestUnassignedFieldNumber keeps counting up. Since FieldInfos allocates an array for number -> FieldInfo, this array will get larger and larger, effectively leaking memory.

We can fix this by resetting lowestUnassignedFieldNumber to -1 in FieldNumbers.clear().

I'll write a unit test and attach a patch.