sqlite as the default CNID backend

[rdmark]

since the sqlite backend has had a handful of release cycles to mature, I thing it's time to start planning for making it the default, and flag the dbd backend as legacy (if not deprecated)

I propose: for the 4.5.0 release series, add documentation and warning logs (when loading the dbd backend) to make users aware that the default is going to change in the future

in the subsequent feature release (be it 4.6.0 or 5.0.0) flip the priority order of dbd and sqlite in meson.build so that the latter gets picked as the default when dependencies for both are available

we also might want to add logic that detects BerkeleyDB files for a volume being loaded and print warning logs advising administrators to rebuild their CNID dbs with dbd -f

[andylemin]

For 5.x, due to the related plans/features, we will definitely need to remove DBD to clear some technical debt that blocks some 5.x features.

Pagination work made this painfully clear, hence why pagination was built in such a way to enable its easy removal.

I expect we will still have a 4.6 release, which will be continued house keeping, declutter, and depreciations in preparation for 5?

[rdmark]

alright, so how about this for a plan:

1. 4.5.0: make is clear in documentation and release notes that this is the final release where dbd is default
2. 4.6.0: sqlite as the default backend, flag dbd as deprecated in documentation and release notes
3. 5.0.0: remove dbd

[andylemin]

Sounds great. I'd like to clear down the issues list in 4.6 too. I've already picked some of them up and working on them for 4.6

[rdmark]

I addressed step 1 in #3008