SPARK-4805 [CORE] BlockTransferMessage.toByteArray() trips assertion

Allocate enough room for type byte as well as message, to avoid tripping assertion about capacity of the buffer

Author: Sean Owen <sowen@cloudera.com>

Closes #3650 from srowen/SPARK-4805 and squashes the following commits:

9e1d502 [Sean Owen] Allocate enough room for type byte as well as message, to avoid tripping assertion about capacity of the buffer

(cherry picked from commit d8f84f26e388055ca7459810e001d05ab60af15b)
Signed-off-by: Aaron Davidson <aaron@databricks.com>
Remove Spillable minimum threshold and sampling