Fix incorrect level assignment during fields parsing

Everytime a sub-selector is encountered, new level is created.
This behaviour is not in accordance to the one described in Javadoc.
Side effects of this issue are missing data in RESTCONF, for NETCONF
mount-points it will fail with NullPointerException because LinkedPathElement is expected on previous level but it cannot be found.
Add proper level tracking and test-cases.
Example:
fields=security:objects(zones/zone);application-identification(user-defined-applications/user-defined-application/app-name)

incorrect structure before:
0 = security:objects, application-identification
1 = zones
2 = zone
3 = user-defined-applications
4 = user-defined-application
5 = app-name

correct structure after:

0 = security:objects, application-identification
1 = zones, user-defined-applications
2 = zone, user-defined-application
3 = app-name

JIRA: NETCONF-660
Signed-off-by: Peter Puškár <ppuskar@frinx.io>
Change-Id: Icaf7c181b23ce9f7110cd6f63f17b55b4c1bc322