Feature/max open files flag and write rates limit

[asdacap]

• Expose max open file settings.
  • Useful to prevent crash under very heavy compaction.
• Expose write rate limit.
  • Useful to limit the effect of compaction writes whose otherwise can't be controlled easily.

Changes

• Extract db settings handling to its own file.
• Expose max open files settings.
• Expose write rate limit settings.

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

Notes on testing

• Open files handle seems to be capped when checking via collectd.
• Write rates limit seems to work also, but not very smooth.