[SPARK-9244] Increase some memory defaults

There are a few memory limits that people hit often and that we could
make higher, especially now that memory sizes have grown.

- spark.akka.frameSize: This defaults at 10 but is often hit for map
  output statuses in large shuffles. This memory is not fully allocated
  up-front, so we can just make this larger and still not affect jobs
  that never sent a status that large. We increase it to 128.

- spark.executor.memory: Defaults at 512m, which is really small. We
  increase it to 1g.

Author: Matei Zaharia <matei@databricks.com>

Closes #7586 from mateiz/configs and squashes the following commits:

ce0038a [Matei Zaharia] [SPARK-9244] Increase some memory defaults