config: separate into static and dynamic parts

[ThomasWaldmann]

see #4422 discussion.

in the repo and also in the cache, we have a ini-like config file that stores some values.

some of them are mostly-static:

• repo in repokey mode: the encrypted key material (only seldomly changed manually) - very important, must not be lost.
• repo parameters (like segment size, segment count per directory) - also quite static, not super important though (can be determined by looking at data/ contents)

some are more dynamic:

• repo last_segment_checked - frequently updated when incremental repo checks are used.
• nonce? check how we store that.

while we have some rather save config-updating code, we want to avoid by all means that some of the rather dynamic updates somehow kills the important config entries, so splitting the config into config and state would be a good thing.