Incorporate multiple network threaded client into FDBDatabaseFactory

[alecgrieser]

The FoundationDB client library starting with version 6.3.11 (see apple/foundationdb#3336 and apple/foundationdb#4269) allows multiple network threads, one per FDB cluster. To allow users to use this through the Record Layer, we should do whatever integration work is necessary to support that. I believe this is just exposing the "client threads per version" network option.

Note that this will depend on getting to updating the FDB client version to 6.3.