Added ToArrayAsync() method and variants that take a hint on the expected size
Added instance LINQ methods on FdbMergeSortSelectable
Added async helpers and refactored TaskHelperes

- FromCancelleation() and FromException() used to return cancelled/failed tasks
- cached default values for tasks
Works even better with the correct types !
Fixed range testing for key space restriction

- If range is "FOO", then from "FOO\x00" up to "FOO\xFF", included!
- Added option on FdbKeyRange.Test() to specify if end should be included or not
- Fixed invered error message when key is outside range
- Updated tests
Fixed/refactored key range and global namespace management

- Added Concat/Extract methods on FdbNamespace and FdbDatabase to add/remove the prefix from Slices
- Added Pack/Unpack mehods on FdbDatabase to add/remove the namespace prefix  when dealing with tuples
- Fixed all unit test to work inside a "T" global namesapce (so as not to interfere with other data in the local db)
Refactored async enumerators and LINQ operators to be easire to compose [WiP]

- AsyncIterator<T> is the base type that handles the state machine
- AsyncFilter<T, R> is an async iterator that takes Ts and outputs Rs
- simplified existing operators to merge sync/async filters/transforms
- rewriten FdbRangeQuery.ResultIterator and PagingIterator to derive from AsyncIterator<T>
- added better support for Take(...)
- added tests