Feature/low priority write

[asdacap]

• When full pruning runs too fast, it will cause some write stalls which will essentially stalls block processing.
• This add a low priority write flag so that full pruning writes will be of low priority so that other writes is not blocked.

Changes

• Add low priority write flags.
• Minor refactor to make read and write with flags consistent.
• Integrate with rocksdb and copy tree visitor.

Types of changes

What types of changes does your code introduce?

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

Testing

Requires testing

• Yes
• No

If yes, did you write tests?

• Yes
• No

Notes on testing

Node write rate limited to 100MBps via cgroup. Seems to work.
Graph is (after, before, after)