Fixed perf regression in FdbConverters and tuple auto conversion

- The 'Converters' dictionnary was losing its KeyComparer when updated
- Removed one of the redundant TypePairComparer class
Fixed failing unit test
Removed unused arguments to ConversionHelper.TryAdaptToString
Added or fixed some XML comments
Bulk: added a Fdb.Bulk.InsertAsync(...) method for processing and writing sequence of items that may exceed the 10MB transaction limit (Work In Progress!)

- Keep items in a pending batch, and tries to commit once the total transaction size size exceeds a threshold (or number of items, or time limit)
- If a batch fails to commit and is retryable, it is split in two halves that are retried (recursively until it commits)