Moved the decision to load p/c fielddata eagerly to a better place.

Closes #5569
Cleanup IndicesFieldDataCache and IndexFieldDataCache

This commit adds several asserts and removes possible `null` values
from the `FieldDataCache` implementation.

Closes #5664
[BUILD] Set -Dtests.jvms=auto by default to make use of multiple JVMs
[TEST] Make BulkTests#testBulkProcessorFlush more robust