- Will *NOT* work with custom allocators, but this code is not meant
   for release anyway.

- turned it to plain list
- reused ptrnode_t list nodes

 - this could happen when reloading

 - Causes issues with synchronization, bug is probably elsewhere,
   reverting before 1.5.0-rc2 release.

This reverts commit 7d9c1744.

Events replan

Conflicts:
	src/knot/zone/events.c

Logging severity

The operation gets ridiculously slow (tens of ops/sec), it is now called
only after a zone flush (certain change, ixfr, update or signing) or
after non-bootstrapping transfer.

partial port of 88e68d90678eeddcdafd56894af1681476032209