[no sq] Server code cleanups / refactors

[sfan5]

depends on #14436, #14423
fixes #14005

To do

This PR is a Work in Progress / Ready for Review.

[lhofhansl]

Can we try to just extract the fix that hangs MT after #14436 and get those in first?
(MinecClone2 is essentially unplayable... Hangs after a few minutes of play.)

I have reverted #14341 and #14436 locally.

[sfan5]

Commits squashed and no longer WIP.
I think the better way of getting the fix in is for someone to just review these PRs.

[SmallJoker]

Compiles.
Singleplayer game showed no issues while testing for a while.
Unittests pass.
Code looks OK.

Found the following with valgrind upon shutdown (closing the window):

==15129== Thread 16 ConnectionSend:
==15129== Syscall param socketcall.sendto(msg) points to uninitialised byte(s)
==15129==    at 0x5471A66: sendto (sendto.c:27)
==15129==    by 0x54EC38: UDPSocket::Send(Address const&, void const*, int) (in ./minetest)
==15129==    by 0x52C462: con::ConnectionSendThread::rawSend(con::BufferedPacket const*) (in ./minetest)
==15129==    by 0x531AF5: con::ConnectionSendThread::sendPackets(float, unsigned int) (in ./minetest)
==15129==    by 0x536BA7: con::ConnectionSendThread::run() (in ./minetest)
==15129==    by 0x6426D7: Thread::threadProc(Thread*) (in ./minetest)
==15129==    by 0x50F3252: ??? (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.30)
==15129==    by 0x53DEAC2: start_thread (pthread_create.c:442)
==15129==    by 0x546FA03: clone (clone.S:100)
==15129==  Address 0x3d06746e is 318 bytes inside a block of size 512 alloc'd
==15129==    at 0x4849013: operator new(unsigned long) (in /usr/libexec/valgrind/vgpreload_memcheck-amd64-linux.so)
==15129==    by 0x522328: std::vector<unsigned char, std::allocator<unsigned char> >::_M_default_append(unsigned long) (in ./minetest)
==15129==    by 0x5180D4: con::makePacket(Address const&, SharedBuffer<unsigned char> const&, unsigned int, unsigned short, unsigned char) (in ./minetest)
==15129==    by 0x51DFA7: con::UDPPeer::processReliableSendCommand(std::shared_ptr<con::ConnectionCommand>&, unsigned int) (in ./minetest)
==15129==    by 0x51EACC: con::UDPPeer::PutReliableSendCommand(std::shared_ptr<con::ConnectionCommand>&, unsigned int) (in ./minetest)
==15129==    by 0x530294: con::ConnectionSendThread::processReliableCommand(std::shared_ptr<con::ConnectionCommand>&) (in ./minetest)
==15129==    by 0x536C92: con::ConnectionSendThread::run() (in ./minetest)
==15129==    by 0x6426D7: Thread::threadProc(Thread*) (in ./minetest)
==15129==    by 0x50F3252: ??? (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.30)
==15129==    by 0x53DEAC2: start_thread (pthread_create.c:442)
==15129==    by 0x546FA03: clone (clone.S:100)
==15129== 

but it doesn't appear that you changed this part. It might be an unrelated issue.