JVM Convenient Transactions API Support in Async Drivers

Summary

The Convenient API for Transactions assumes a synchronous API or at least language support for asynchronous operations (e.g. await). As a result, it's difficult to use transactions currently with either the Scala or the Reactive Streams drivers.

This epic will track the work to create an easier-to-use API for transactions for our async JVM drivers. To do so might involve taking a dependency on a reactive framework, e.g. Reactor 

This epic depends on JAVA-3703, since in that epic we will determine our dependency on a reactive framework.

Documentation

[Scope Document|]
[Technical Design Document|]