Bitcask should check for missing hintfiles on close and write them out.

[evanmcc]

If your hintfiles are deleted, and you shut down cleanly the node before any merges happen, your hintfiles will not be recreated. This is undesirable on very large nodes, so since we already have the hintfile information, we should make sure that it is up to date on bitcask:close().

[angrycub]

Would it be possible to write out the hintfile as soon as the datafile is read rather than waiting on a bitcask:close()? This would prevent a nodes crashing before a proper close or merge from having to fully read the datafile again.

[jonmeredith]

Could make the fold_keys function regenerate it when in recovery mode based on config.