this fixes a problem when a module was removed, but pending
queries referenced it, causing a crash. usually when the
server was busy and a module was unloaded.
as we don’t need to copy layers at all, they’re just iterated
from the array of modules using a macro

before only current I/O handle was tracked for each subrequest, but the
requests may have outlived it (connect request isn't cancelable for
example), if such request terminated after the request was finished,
it checked the address of potentially stale task

this applies for Linux with recvmmsg() support only

when the timeout event was fired, it started another step in query
resolution, but there may have been pending events on previous handle
if the events on the previous handle completed, they would check the
memory of their task (which may have been non-existent) and possibly
double-close the handle

no need to scramble queries satisfied from cache

an average request from cache requires ~12k, an average iterative request 16-64k
this reduces allocation cost when there are now pools on the freelist

since the structures are ~ same size as I/O requests (udp_send_t being 60B larger than others), they all share the same freelist

the stats module keeps track of frequent iterative queries using
probabilistic sampling. the list includes subrequests.
it is useful for tracking over time, prefetching and anomaly reporting

enumerated metrics are always present (set to 0 if not used), this fixes
missing values when not measuring and provides a faster access to common
data