Bug 1198: Make URLSchemaContextResolver thread safe

Make YangModelParser interface deprecated as recommended methods are in
YangContextParser.
yang-parser-impl - Introduce dependency on findbugs for documentation.
URLSchemaContextResolver - first round of refactoring, add
synchronized everywhere where data is changed.

Change-Id: Id53f165740301d085ad8374fc02897a0a19faa38
Signed-off-by: Tomas Olvecky <tolvecky@cisco.com>