Fix comments on data checksum cost settings

Fix comments on data checksum cost settings

The cost parameters for the data checksums worker can be updated by the
user issuing a repeated enable checksum command, but the comments on the
struct members hadn't been updated to reflect this and were out of date.
Another part of the same comment needed better wording to be readable.

Also wrap the reading of the parameters in a lock, there is no live
bug due to not using a lock but it's still the right thing to do.

Author: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Reported-by: Heikki Linnakangas <hlinnaka@iki.fi>
Discussion: https://postgr.es/m/2176020b-ecbc-438b-9fc3-9c3593d9e6fc@iki.fi