daemon/zimport: close transaction after importing batch

daemon/zimport: close transaction after importing batch

I'm really sorry about this.  It's my regression in 5.5.0 (!1225)

Practical consequence was that the RW transaction was held open
until that instance did something with cache (and thus closed),
so any other instance would be frozen in the meantime if doing
anything non-read-only with cache (e.g. startup).
https://lists.nic.cz/hyperkitty/list/knot-resolver-users@lists.nic.cz/thread/6DOXXOA6ACEUBVYPUY3T2MLGIHWOMV6M/