DB garbage collector

[bamidev]

There still needs to be a 'garbage collector', which will remove old blocks, files & objects if the configured amount of 'space limit' is exceeded.

There are two limits.
recent_persistence_limit
prolonged_persistence_limit

The first limit describes how many things are to be stored no matter what.
The second limit describes how many things are to be stored if the configured capacity allows it.
This also means that anything beyond the second limit will be deleted.
The second size can also be unset, in which case your node will try to store all of it.

Defaults:
Objects:
recent_persistence_limit = 1000
prolonged_persistence_limit = 1000000
Files:
recent_persistence_limit = 1000
prolonged_persistence_limit = 100000
Blocks:
recent_persistence_limit = 10MiB
prolonged_persistence_limit = 90MiB

A 'recent' file is a file that is used in a recent object, and a 'recent' block is a block that is used in a 'recent' file.
The block limits are not about the number of blocks, but about the actual size of them.