Do not cache path from root
Heap analysis shows that full 16% of SchemaContext memory is retained
by SchemaPath instances. We can reduce this by removing path caching,
so that each query to getPathFromRoot() results in a new collection.
This saves a pointer field in the structure, saving 8 bytes in typical
64bit JVM scenarios -- going from 32/48 to 24/40 bytes, saving 16-25%
instance size.
JIRA: YANGTOOLS-1076
Change-Id: I5613764c513b9b54473e2e587de2b8b38713ed15
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>