Use sbt.Sync to copy jars

[jroper]

Currently multijvm just does a straight copy of jars from the classpath list to the jars directory. The problem with this is that when the classpath changes, any jars that shouldn't be there are left there. In a project where dependencies are changing frequently, this leads to frequent unpredictable LinkageErrors, unless you ensure you do a clean before each test run, which slows everything down.

sbt.Sync will both speed things up because it will avoid unnecessary copying, and it will ensure the classpath is correct even when the classpath changes, avoiding spurious LinkageErrors.

[ghost]

This catches each new developer on our project. Please fix this as part of a ticket under [email protected]