Remove old schema file code, use strict Schema parsing

See merge request memri/pod!223

Add _edge _source _target props for edge items

See merge request memri/pod!221

Update source item on edge insertion

See merge request memri/pod!220

# Conflicts:
#	src/internal_api.rs

Allow creating edges with already existing self_id

See merge request !219

Edge search properties

See merge request memri/pod!218

Add support for search by arbitrary properties

See merge request memri/pod!217

Update start plugin

See merge request memri/pod!214

Allow search in bulk

See merge request memri/pod!213

added kubectl in dockerfile

See merge request memri/pod!216

Combine edge expansion with /search API

See merge request memri/pod!212

* Rename StartPlugin -> PluginRun
* Rename PluginRun.container -> PluginRun.containerImage
  This terminology is technically better as docker runs
  container _images_ in _containers_.
* Reset database (via name change) due to breaking
  change in migration files content.
  (This could be avoided, but no need for now.)
* Update v3 -> v4 because a rename is a breaking change

Update CONTRIBUTING doc

Closes #387

See merge request !211

Ignore non-harmful schema overrides

See merge request !210

Add more details to Schema validation failures

See merge request !209

Document Schema synchronization process

See merge request memri/pod!205