CHT: Add insert_and_get method

With upcoming G1 remembered set changes (JDK-8017163) which extensively use the CHT it would be advantageous to have a combined insert_and_get() method to decrease the amount of global synchronization needed as after inserting a node the code always works with it.

I.e. instead of

bool found = cht.insert(...)
[do something if found]
cht.get(..., found);
return found.value();

it would be nice to avoid the second global synchronization by

bool found = cht.insert_and_get(..., found, ..);
[do something if found]
return found.value();