BUG-4688: Rework SchemaContext module lookups

BUG-4688: Rework SchemaContext module lookups

SchemaContext.findModule*() family of methods is making the mistake
of treating null revision as a wildcard. This does not match RFC6020
and RFC7950, which allow for modules without a revision, hence we need
to mark non-existing modules with a magic constant, which maps to
1970-01-01.

While we can use identity checks and assume such a revision will not
be seen in the wild, this approach does not really work with maps
and creates confusion with users, who need to treat DEFAULT_DATE_REV
as something special.

This patch cleans up the lookup methods and makes
ModuleIdentifier.getRevision() return an Optional<Date>, in preparation
to making the transition to SourceIdentifier.

As a side-effect we switch to using QNameModule-based lookups in couple
of places, which are significantly faster than combined name/revision
lookups.

Change-Id: I5602192e26d721d2461248c612d98a1b305c6958
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>