Refactor reader-file handling in StdFileManager

[ecton]

The current implementation of read-file caching isn't great -- it never closes any files. This is fine for now, but we'll eventually run into open-file limits.

The read-files (and maybe write-files) should be switched to an Lru cache to automatically close file handles that haven't been needed as we need to open new files.

[ecton]

I noticed yesterday that this is causing open file issues on Mac OS when running the core-suite unit test in BonsaiDb. That test, with all features enabled, runs 99 unit tests, each which create a database with multiple views. The default max-open-files limit on Mac is only 256.