Add TimeStampProvider support

[Drew]

Adding Support for a mechanism similar to ITimestampProvider in Akka.Persistence.Sql.Common will be useful to some users that wish to have timestamps in a format other than .NET DateTime Ticks (i.e. Unix epoch instead)

Ideally, we should try to use a base/abstract class instead of an Interface, or at least benchmark the difference between the two; Reason for this is on high velocity systems, cost of interface calls on reading/writing events may have an impact instead of a simple virtual call.

[David Cumps]

Or for people wanting to use Noda Instant types