unref or join some threads

avoid leaving thread state and stacks on heap after threads exit
plugins: echotest and streaming: use atomic operations for stopping and initialized flags

... where most critical

use _try variant of g_thread_new for watchdog, since we're checking for failure

Stop watchdog thread before freeing sessions table
fix minor mixup of sws and admin_sws
sctp / dtls threads should deref themselves so they are cleaned up

since no other thread join()s them
Merge branch 'thread_stack_leaks' of https://github.com/ploxiln/janus-gateway into ploxiln-thread_stack_leaks
Further changes to the other threads (plugin and core)
Merge branch 'ploxiln-thread_stack_leaks'