Improve crash handling

[Kleissner]

I read the code and documentation and wanted to ask if there is a specific reason why you are discarding the old index files and always recreating them? It sounds like a dangerous default and expensive especially re production environments.

In the event of a crash caused by a power loss or an operating system failure, Pogreb discards the index and replays the WAL building a new index from scratch. Segments are iterated from the oldest to the newest and items are inserted into the index.

My use case is to store billions of key-values - and if I read the code correctly, anytime it crashes for any reason, the lock file will be detected and causes Pogreb to discard the index files (*.pix). Current estimated indexing time is 8 days and likely hundreds of GB. Any reboot/crash to cause reindex of hundreds of GB and days of work doesn't make sense? Possible solutions:

1. New Options.ReindexOnCrash to allow the user to specify whether (on false) it should try to re-open, or (on true) immediately reindex everything; or instead:
2. Introduce Options.AutoReindexCorruptDatabase which triggers a reindex only in case openIndex returns an error. The lock file will be disregarded for crash detection and it will always try to open the existing database.

I believe the second option makes most sense. In case of crashes most if not all users assume the database will just pick up where it left - especially in production environments.