Optimize isConfiguration()/isIgnoringConfig() interplay

Optimize isConfiguration()/isIgnoringConfig() interplay

Now that we have coalesced these two states into a flag field,
their interplay is more apparent -- isIgnoringConfig() is always
checked before isConfiguration() and if it is set, it also implies
isConfiguration() is also set.

Using flags allows us to express this implication by simply setting
the right bits. This results in fewer flag operations performed and
a more direct dispatch between methods.

isConfiguration() is also improved to access parent.isConfiguration()
only when necessary, improving performance when a config substatement
is present.

JIRA: YANGTOOLS-652
Change-Id: Ifb18ee16e33e93de4300cc80e4ba67a9ae40bbde
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>