Issue when device storage is less than rhizome.min_free_space

[JakobPennington]

I have been trying to send messages from Serval-DNA from a Dragino to my smartphone. No errors are reported in shell, however messages are not getting through. As it happens, the Dragino has less storage than the amount of space Serval-DNA tries to keep free (through rhizome.min_free_space in the config, which is set by default to 100MB). It seems the rhizome database is cleaned up in an attempt to keep this space free.

Setting min_free_space to 0 resolves this issue, however it would be handy if the total free space is checked when creating/modifying the database, and if it's less than min_free_space, this is reported through an error.