BUG-7464: Add MainNode.trySize()

BUG-7464: Add MainNode.trySize()

Having the ability to estimate size will be useful for Spliterators,
add a method for reading the currently-cached value without actually
computing it.

Also rename cachedSize() to size() and make it enforce being called
with an immutable snapshot. For CNodes this allows us to get rid
of atomic access to the size field, as the order of updates and
visibility propagation of size does not matter -- all concurrent
computations will arrive at the same result. The worst that can
happen we spend more time computing computing the value.

Change-Id: I84e33d96c7aa6f97cb33e0d1c31178916541c693
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>