Redefine the concept of the database migration.

* Make it fit for running incremental upgrades.
* Migration is specific to the document type not the version of the
  application.
* Define IMigration to have synchronous and asynchronous part.
* Logic for finding the shortest path during the migration.
* feat-dbload script now detects and performs incremental upgrades,
  changing only the documents which need changing.
Merge branch 'master' into release-0.28.x
Remove deprecated export and migration agents.

They never got to be used, currently they are to be considered rotten bits.
Add a convinient helper for using DocumentByType view.
Fix incorrect usage of IVersionDocument.has_migrated
Implement a special tagging of the snapshot of the versioned document
which prevents the adaptation from being made.

This is usefull for loading in only the parts of the document from the
view index.
Fix wrong parameter.
Fix migration script promoting non-versioned to versioned documents.
Make it possible to decide about the document deletion from the
synchronous_hook() of the migration.
Add test case for inheritance of enums (to be sure how it works).
Implement a public function to compare the document content ignoring
the metafields.
Do not overwrite design documents from feat-dbload.

* Debug verbosily about what changes are to be deployed.
* This responsibility is to be moved to "feat-service upgrade" command.
Implement IDatabaseClient.copy_document() method.

This is necessary to implement the live view update.
Implement "feat-service upgrade" operation.

Update the couchdb views, wait for reindex them, and restart.
Add pushing initial data to the responsibilities of feat-service upgrade method.# Please enter the commit message for your changes. Lines starting
Add an option to import specified module from feat-service upgrade.
Add a service command cleaning up stale alert services.
Merge branch 'master' into release-0.28.x
Fix typos in log message bodies.
Handle explicitly the errors which might arise while querying the index of
the query view.