Throttle vacuum workers based on chunk age

[jgpruitt]

Description

Throttle vacuum workers based on chunk age. Figure out how many workers to used based on the oldest transaction id age of the chunks in our batch. The age should be between vacuumFreezeMinAge and autovacuumFreezeMaxAge. The closer the maxChunkAge is to the autovacuumFreezeMaxAge, the more workers we assign. If maxChunkAge is == vacuumFreezeMinAge, then we just use 1 worker.

	       vacuumFreezeMinAge
	        |              maxChunkAge
	        |               |       autovacuumFreezeMaxAge
	0-------|---------------X--------|------
	        1000            6789     11000

In this example, maxChunkAge is 57.89% of the way between vacuumFreezeMinAge and autovacuumFreezeMaxAge. If maxParallelism is 5, then we will use 3 workers.

If the autovacuum engine beats us to a chunk, skip that chunk.

Merge requirements

Please take into account the following non-code changes that you may need to make with your PR:

• CHANGELOG entry for user-facing changes
• Updated the relevant documentation

[cevian]

Overall looks good, but would like some cleanup esp wrt using current_setting

[cevian]

LGTM but I'd like some of Arun's comments addressed. Especially the ones relating to metric exposition