Disable info log by default and add infoLogLevel option

[vweevers]

Closes #113.

Benchmarks show that this doesn't improve write performance (with the info log enabled, there's a small amount of logs being written on every compaction). It does however fix the destroy() bug on Windows in a cleaner way, and prevents endlessly growing logs that you didn't ask for (cc @MeirionHughes).

git checkout disable-info-log
level-bench run write -b [-n 2e6 --concurrency 10]
level-bench run write -b [-n 2e6 --concurrency 10] --db [--infoLogLevel debug]
level-bench plot write

[vweevers]

@ralphtheninja @peakji Is either of you able to review this week or next week? This PR is the last thing I want to get in before releasing 4.0.0 (disabling the info log could be considered a breaking change).