Add option to disable low priority writes for full pruning

[asdacap]

• Add option to disable low priority writes for full pruning.
• With 16GB mem budget, reduce full pruning time from 2:10 to 0:48, but will definitely stall block processing.
• Originally I wanted to have configurable db tune, but it does not do much compared to just disabling low priority writes or letting the ssd cool down and probably just complicate things. Perhaps the semi-ordered insert kinda already reduce write amplification somewhat.

Scenario	Low priority write	Time	Total Write (TB)	Block Processing AVG	Block Processing P50	Block Processing P90	Block Processing P99
Baseline block processing		-		359	336	539	711
Default	yes	2:10	2.30	613	580	860	1260
StableWrite	yes	1:30	1.92	898	822	1280	2280
WriteBias	yes	0:54	1.67	1680	1130	3470	9470
HeavyWrite	yes	0:43	1.54	5000	19200	12500	12500
AggesssiveHeavyWrite	yes	0:39	1.08	7410	4190	12500	12500
Default	no	0:48	2.00	12500	12500	12500	12500
StableWrite	no	0:46	1.80	12500	12500	12500	12500
WriteBias	no	0:36	1.68	4370	3440	9550	12500
HeavyWrite	no	0:38	1.40	8700	5800	12500	12500
AggressiveHeavyWrite	no	0:41	1.05	6710	4370	12500	12500

Changes

• Add option to disable low priority writes.

Types of changes

What types of changes does your code introduce?

• New feature (a non-breaking change that adds functionality)

Testing

Requires testing

• Yes
• No

If yes, did you write tests?

• Yes
• No

[smartprogrammer93]

LGTM