It is safe because re-sending still works during graceful shutdown as
long as resolving takes place (and resolve tasks set their own timeouts
that are not cancelable).

Also, the new ctx->cancelable flag can be set to make resolve task's
timeout event cancelable.

No functional changes.

Notably, on ppc64 with 64k pagesize, slab 0 (of size 8) requires
128 64-bit elements for bitmasks.  The code bogusly assumed that
one uintptr_t is enough for bitmasks plus at least one free slot.

Resolving an SRV record includes resolving its host names in subrequests.
Previously, if memory allocation failed while reporting a subrequest result
after receiving a response from a DNS server, the SRV resolve handler was
called immediately with the NGX_ERROR state.  However, if the SRV record
included another copy of the resolved name, it was reported once again.
This could trigger the use-after-free memory access after SRV resolve
handler freed the resolve context by calling ngx_resolve_name_done().

Now the SRV resolve handler is called only when all its subrequests are
completed.

The flag indicates that the resolve handler is called asynchronously after the
resolve function ngx_resolve_name()/ngx_resolve_addr() exited.