* VuFindSearch/Backend/Solr/Backend.php (aggregateSpellcheck): New
  function. Factored out from search().
  (search): Factored out aggregated spellcheck, use local boolean var to
  indicate spellcheck.

* VuFindSearch/Backend/Solr/Backend.php (search): Set offset and limit
  in parameter bag.
* VuFindSearch/Backend/Solr/Connector.php (search): Change function signature,
  offset and limit contained in parameter bag.

* VuFindSearch/Backend/Solr/Connector.php (terms): New method. Extract
  index terms.
* VuFindSearch/Backend/Solr/Backend.php (terms): Use connector method.

Eliminated getRecord/getRecords in Results classes.

* VuFindSearch/Backend/Solr/Backend.php (delete): Removed.

* VuFindSearch/Backend/Solr/Backend.php (injectResponseWriter):
  New method. Inject response writer.
  (alphabeticBrowse, terms, similar, retrieve, search): Use new
  method.

* VuFindSearch/Backend/Solr/Backend.php (search): Update docstring.

* VuFindSearch/Backend/Solr/Backend.php (deserialize): Log SOLR query
  time.

* VuFindSearch/Backend/Solr/Response/Json/Terms.php: New
  class. SOLR term component output.
* VuFindSearch/Backend/Solr/Backend.php (terms): New
  function. Return list of terms.
* VuFindSearch/Backend/Solr/Connector.php (getTerms): Removed.
  (query): Declare public. To be called by Backend.

* VuFindSearch/Backend/BackendInterface.php (similar):
  Removed. Defined by MoreLikeThis feature interface.
* VuFindSearch/Backend/Solr/Backend.php: Implement feature
  interface.
* VuFindSearch/Feature/MoreLikeThis.php: New file. MoreLikeThis
  feature.

* VuFindSearch/Backend/Solr/Backend.php: Remove
  WritableBackendInterface.
* VuFindSearch/Backend/Feature/WritableBackendInterface.php:
  Removed.

* VuFindSearch/Backend/Solr/Connector.php (delete): Implement
  delete() by directive.
  (update): New method. Send document to update request handler.
* VuFindSearch/Backend/Solr/Backend.php (delete): Modify function
  signature of delete() operation.

* VuFindSearch/Backend/Solr/Connector.php ($lastQuery): Renamed
  from $lastRequest.
  (getLastQueryParameters): Dto.
  (resubmit): Dto.
  (query): Dto.
* VuFindSearch/Backend/Solr/Backend.php (search): Dto.

* VuFindSearch/Backend/Solr/Backend.php (update, deleteAll)
  (delete): Define methods to fullfill WritableBackendInterface.

* VuFindSearch/Backend/Solr/Backend.php (__construct): Use null as
default backend identifier.

* VuFindSearch/Backend/Solr/Backend.php (__construct): Use object hash
as default identifier.
* VuFindSearch/Backend/Solr/Backend.php (__construct)
(setIdentifier): Inject backend identifier via setter, not
constructor.
* VuFind/Search/BackendManager.php (onResolve): Inject backend
identifier.
* VuFindSearch/Backend/BackendInterface.php (setIdentifier): Make
setIdentifier() part of the backend interface definition.
* VuFind/Search/Factory/SolrReservesBackendFactory.php (__construct):
  Dto.
* VuFind/Search/Factory/SolrDefaultBackendFactory.php (__construct):
  Dto.
* VuFind/Search/Factory/AbstractSolrBackendFactory.php
($serviceName): Removed. Name injected by BackendManager.
* VuFind/Search/BackendManager.php (onResolve): Inject backend
  identifier.

* VuFindSearch/Backend/Solr/Backend.php (deserialize): Throw
  BackendException, instead of RuntimeException.
  (createRecordCollection): New function. Create RecordCollection
  from serialized JSON.
  (search): Re-implement spellcheck and spellcheck merging.
* VuFindSearch/Backend/Solr/Connector.php (resubmit): Accept
  parameters for resubmit as argument.

* VuFindSearch/Backend/Solr/Backend.php: Modify function signature
according to BackendInterface.
(search): Adjust check of spellcheck enabled/disabled.