move readside to "projections with an actor"

[zenyui]

Is your feature request related to a problem? Please describe.

• Our current read side does not need JDBC access for the handler method (it does need to commit to the JDBC offset store, though).
• We would like to implement backoff (Readside exponential backoff #329 ) and the current JDBC read handler would lock the thread, which would rob the write side of a thread and slow down writes (bad!)

Reference ticket #318

Describe the solution you'd like
Perhaps move us to implementing the projection handler as an actor (see doc)

Describe alternatives you've considered
n/a

Additional context
The akka doc assumes you store offsets in cassandra, but it should be easy to use the JDBC offset store instead. We can pair if you need help.

[zenyui]

@klvmungai I heard you wanted a fun task :)