Remove usage of MutableValidationReport where possible

[James Navin]

Most usages of the MutableValidationReport can be changed to merge operations (e.g. map/reduce with merge) and use of the ValidationReport#singleton() factory method.

Ideally we could remove as much of the report mutation as possible.

This may mean some work to make ValidationReport#merge more efficient - e.g create a view of the merged reports instead of creating a new list to store the messages.

[James Navin]

Use of the MutableValidationReport within a single method is fine. I would like to remove the pattern of passing a mutable report around to collect errors.

[James Navin]

Most of the work for this has been done as part of https://bitbucket.org/atlassian/swagger-request-validator/pull-requests/25 (big thanks to @sdoeringNew).

The only usage of MutableValidationReport that remain are in the ValidationReport merge operations and the ValidationReportFormatterTest. Issue #35 aims to improve the merge operations to avoid the use of the MutableValidationReport, which will allow this issue to be closed.

[James Navin]

Available in v1.0.16 - thanks to @sdoeringNew