Retry.withWait should use scala.concurrent.blocking

[tabdulradi]

Hello all,

Thanks all for this lib, keep it up!

Retry.withWait could wrap the Thread.sleep inside a blocking block, which warns the global execution context (so that it might add more threads).

Also I'd appreciate make it clearer in the docs that the method blocks a Thread.

[loicdescotte]

Thanks we will look at this!