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/

NEWS

@@ -5,6 +5,7 @@ Bugfixes
 --------
 - modules/dns64: fix incorrect packet writes for cached packets (#727, !1275)
 - xdp: make it work also with libknot 3.1 (#735, !1276)
+- prefill module: fix lockup when starting multiple idle instances (!1285)
 
 
 Knot Resolver 5.5.0 (2022-03-15)

daemon/zimport.c

@@ -426,6 +426,7 @@ static void zi_zone_process(uv_timer_t *timer)
 	kr_timer_start(&stopwatch);
 
 	int ret = trie_apply(z_import->rrsets, zi_rrset_import, z_import);
+	(void)kr_cache_commit(&the_worker->engine->resolver.cache); // RW transaction open
 	if (ret == 0) {
 		kr_log_info(PREFILL, "performance: validating and caching took %.3lf s\n",
 			kr_timer_elapsed(&stopwatch));