[C#] Remote subkv fixes (#542)

* Add client side support for SubscribeKV

* basic working impl

* Separate span byte serializer into client and server

* Added support in Subscriptions. TODO: prefix matching for subscriptions, publish deletes to keys (#499)

* Added support in Subscriptions. TODO: prefix matching for subscriptions, publish deletes to keys

* Added prefix matching, but similar to redis and unoptimized

* Added null checking for subscriptions on removeSubscriptions

* Made changes to correct concurrency and type checking

* Optimized and corrected the Start() method and Publish() method in publish call

* Added prefix subscriptions. Added a new client serializer call, need to discuss

* Corrected few nits

* Cleanup of code, and resolving NIE

* Nit fix of null checking

* Fixed small nit in MemoryparamSerializer to return Memory<byte> of correct length, and in publish call corner case

* Fixed small nit in MemoryparamSerializer to return Memory<byte> of correct length, and in publish call corner case (#511)

* Resolving merge conflicts

* Removing the subscriptions belonging to a session on disconnection

* Added unit tests and VarLenClient test

* nit fix

* Made broker associated with FasterServer and not FasterKVProvider

* fixed a bug for large values in pub-sub

* Changed the SubscribeKVBroker to be spawned by the user and passed to provider

* fixed nit

* Merged with recent master code

* Checked fixed len client

* Fixed bug in Write of key in server serializer

* Added null check for broker

* Fixed a nit: subscriptions dictionary should contain key as sid and val as serversession

* fixed nit

* Removed old ByteArrayComparer

Co-authored-by: Badrish Chandramouli <badrishc@microsoft.com>