RFR: 8221392: Reduce ConcurrentGCThreads spinning during start up

[Per Liden]

During startup, ConcurrentGCThreads end up in a loop sleeping for 1ms 
over and over (typically 30-50 times), until is_init_completed() is 
true. This is not optimal, and we should instead have them wait until 
they are signaled. This problem has been observed before, as comments in 
JDK-8136854 suggests.

This patch introduces wait_init_completed(), which will block until 
set_init_completed() is called. Note that the newly introduced 
InitCompleted_lock is only used to control the interaction between 
wait_init_completed() and set_init_completed(). is_init_completed() is 
still lock-free and was just cleaned up to use proper atomic 
load_acquire/release_store semantics.

Bug: https://bugs.openjdk.java.net/browse/JDK-8221392
Webrev: http://cr.openjdk.java.net/~pliden/8221392/webrev.0

/Per