Rework EXPLAIN format for incremental sort

Rework EXPLAIN format for incremental sort

The explain format used by incremental sort was somewhat inconsistent
with other nodes, making it harder to parse and understand. This commit
addresses that by

 - adding an extra space to better separate groups of values

 - using colons instead of equal signs to separate key/value

 - properly capitalizing first letter of a key

 - using separate lines for full and pre-sorted groups

These changes were proposed by Justin Pryzby and mostly copy the final
explain format used to report WAL usage.

Author: Justin Pryzby
Reviewed-by: James Coleman
Discussion: https://postgr.es/m/20200419023625.GP26953@telsasoft.com