Fix replConfig merging and evaluate values in Config.fromHadoop

Adds `Config.defaultFrom` which will correctly merge Config.default with existing values in JobConf (keys besides "io.serializations" can be added as needed), and use this in `replConfig` so we pick up the correct values in Hadoop mode.

Also had to fix ReplTest because the Execution[T] changes had made it so it was only ever running in Local mode. Because ReplImplicits.mode is a mutable var which gets picked up dynamically, the repl tests have to be run sequentially (for now).
actually, can we do this just once? We might need to make a fresh jar for every run to ship to the mappers. I'm not 100% sure, but the safest course might be to have config be the way you have it, but a replConfig or executionConfig that is a def that adds the jar on every call.

This is just anxiety on my part. I really don't understand the createReplJar mechanism or what happens if it is not there.