feat!: Open database should be a folder

[rkuris]

We have several use cases for keeping all database files in a folder at this point:

• RootStore wants one or more separate files
• Deletion log should be in an append-only file Deleted node log #832
• Database chunking or freezing a section of the database to make copying easier

This issue simply changes the parameter to open to require it to be a directory instead of a file, and fixes all the tests and API callers both internally and externally.

[alarso16]

Do we want the log file to be created within this folder too?

[RodrigoVillar]

With ava-labs/coreth#1416 merged in, this should be easier to do as we now store all Firewood related content in the firewood folder.