feature(parser): consolidation of multiple HTTPRouteMatch objects into single kong.Route

[jrsmroz]

What this PR does / why we need it:

This PR consolidates HTTPRouteMatch objects into paths in kong.Route if they share the route, filters and matching data.

The behaviour depending on CombinedRoutes being enabled or not:

• When the CombinedRoutes feature is disabled, the parser generates one kong.Service per HTTPRouteRule and one kong.Route per HTTPRouteMatch pointing to that service.
• When theCombinedRoutes feature is enabled, prior to this PR, multiple HTTPRouteRule objects can be consolidated into a single kong.Service if they share the backend refs. However, single HTTPRouteMatch generates a single rule pointing to the generated service.
• With this PR and the CombinedRoutes enabled, the multiple HTTPRouteMatch objects are further consolidated into a single kong.Route with multiple paths.

The following limitations apply to the consolidation logic:

• HTTPRouteMatch objects cannot be consolidated if they belong to different HTTPRoute objects.
• HTTPRouteMatch objects cannot be consolidated if HTTPRouteRule objects they belong to have different backend refs.
• HTTPRouteMatch objects cannot be consolidated if HTTPRouteRule objects they belong to have different filters.
• HTTPRouteMatch objects cannot be consolidated if they have different spec (headers, query params, method).
• The different paths does not prevent consolidation of HTTPRouteMatch objects.

This PR changes the code path used for generating kong.Route and kong.Service objects when service based consolidation is enabled, added previously by #3008. It does not change the behaviour of soon to be deprecated kong.Route and kong.Service generation used when service based consolidation is disabled. The PR extracts common functions from this code path.

Which issue this PR fixes:

Part of #2881

Special notes for your reviewer:

This feature does not yet provide a benchmark, to measure performance hit, but it's intended to provide benchamarks too.

PR Readiness Checklist:

Complete these before marking the PR as ready to review:

• the CHANGELOG.md release notes have been updated to reflect any significant (and particularly user-facing) changes introduced by this PR

[jrsmroz]

@pmalek I propose we also wait for Travis to take a look. There is no rush in merging.

[rainest]

LGTM, my comments are just on documentation stuff.

[jrsmroz]

@rainest Applied the changes after the review. PTAL.