Adding a ValueGenerator to the Redis provider

Fix for #651 
Moved the common parts of SqlServerSequenceValueGenerator out into BlockOfSequentialValuesGenerator and have both SqlServerSquenceValueGenerator and RedisSequenceValueGenerator inherit from that.
I was trying not to get into async yet. I have another issue coming up to support the needed async APIs in Redis and was waiting for that - but this inherits from BlockOfSequentialValuesGenerator - which needed an async copy of the API to also support SqlServerSequenceValueGenerator.

That said your suggestion should be a small fix. I'll try it and see if it causes any issues. If not I'll update.