Backport netmgr-related merge requests

This issue contains a list of netmgr-related merge requests which should be eventually backported to v9_16, but may need to wait in a queue for a while before that happens.

Merge requests that must be backported:

• !3781 (merged) Fix socket closing races.
• !4318 (merged) (included in !4455 (merged)) Resolve "Add netmgr functions to support outgoing DNS queries"
• !4341 (merged) (included in !4455 (merged)) Fix improper closed connection handling in tcpdns.
• !4386 (merged) (included in !4455 (merged)) Turn all the callback to be always asynchronous
• !4444 (merged) Refactor netmgr and add more unit tests
• !4452 (merged) (included in !4455 (merged)) Avoid netievent allocations when the callbacks can be called directly
• !4458 (merged) (included in !4455 (merged)) Make netmgr initialize and cleanup Winsock itself
• !4459 (merged) (included in !4455 (merged)) Distribute queries among threads even on platforms without SO_REUSEPORT_LB
• !4465 (merged) (included in !4455 (merged)) Don't use stack allocated buffer for uv_write()
• !4468 (merged) (included in !4455 (merged)) Fix datarace when UDP/TCP connect fails and we are in nmthread
• !4469 (merged) (included in !4455 (merged)) Use sock->nchildren instead of mgr->nworkers when initializing NM
• !4472 (merged) (included in !4455 (merged)) Fix s/HAVE_REUSEPORT_LB/HAVE_SO_REUSEPORT_LB/ typo in #define

Merge requests that may be backported:

• !4115 (merged) Resolve "convert dig and friends to use the netmgr"
• !4246 (merged) use netmgr for xfrin
• !4374 (merged) address some possible shutdown races in xfrin
• !4397 (merged) Resolve ""dig" crashes when interrupted while waiting for a TCP connection"
• !4466 (merged) Configure the system-wide TCP connection timeout on OpenBSD
• !4633 (merged) Resolve "Incorrect size passed to isc_mem_put"
• !4628 (merged) Improve reliability of the netmgr unit tests
• !4845 (merged) netmgr: Make it possible to recover from ISC_R_TIMEDOUT (backported without the relevant changes to dig, rndc, or xfrin)
• !4898 (merged) Prevent the double xfrin_fail() call
• !4930 (merged) ensure read timeouts are recoverable
• !4796 (merged) Add workaround for "nslookup segfaults for SERVFAIL"
• !4918 (merged) Refactor taskmgr to run on top of netmgr
• !4983 (merged) Destroy netmgr before destroying taskmgr
• !4981 (merged) Add nanosleep and usleep Windows shims
• !4982 (merged) Add support for generating backtraces on Windows
• !5009 (merged) Bump the netmgr quantum to 1024
• !5013 (merged) initalise sock->cond
• !5021 (merged) Fix the outgoing UDP socket selection on Windows