DatabasePool::set_schema() should checkout the database before trying to modify the atomic schema
DatabasePool::set_schema() should checkout the database before trying to modify the atomic schema

Once locked, if, and ONLY IF, the current scheme hasn't yet changed
from the one used to generate new_schema, it updates the atomic schema
and writes it to the database, otherwise return "false", so the new
schema can again be recalculated from the new atomic schema.

Kronuz/Xapiand,Kronuz/Xapiand,Kronuz/Xapiand,Kronuz/Xapiand,Kronuz/Xapiand,Kronuz/Xapiand