Rename default backend identifier to `VuFind'

* VuFind/Search/Factory/SolrDefaultBackendFactory.php
  (createService): Rename default backend identifier to `VuFind'.
Retrieve and lazy-load search service

* VuFind/Search/Solr/Results.php ($searchService): New
  property. Search service.
  (getSearchService): New function. Retrieve and lazy-load search service.
Move QueryAdapter to VuFind namespace

* VuFind/Search/Legacy/QueryAdapter.php: Moved from VuFindSearch
  namespace.
Return AbstractQuery object, required for new search service

* VuFind/Search/Solr/Params.php (getQuery): New function. Return
  AbstractQuery object, required for new search service.
Fix, set client method after check parameter length

* VuFindSearch/Backend/Solr/Connector.php (sendRequest): Fix, set
  client method after check parameter length.
Expose spellcheck information

* VuFindSearch/Backend/Solr/Response/Json/RecordCollection.php
  (static): Add spellcheck to response template.
  ($spellcheck): New property. Spellcheck information.
  (getSpellcheck): New function. Return spellcheck information.
Modify NormalizeSolrSort listener to work with SOLR parameters

* VuFind/Search/Listener/NormalizeSolrSort.php (onSearchPre): Modify
  to work with raw SOLR parameters.
Various small fixes

* 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.
Create search parameters in QB::build(), only set hl.q if hl is on

* VuFindSearch/Backend/Solr/QueryBuilder.php (createSearchParams):
  Removed. Code moved to build().
  (build): Create search parameters, only set hl.q if hl is on.
fixup prev
New function: More like this; allow parameters for delete

* VuFindSearch/Service.php (similar): New function. More like
  this.
  (delete): Allow passing of backend parameters.
Rename default backend to Solr
Set highlighting and hidden filters in Backend factory prototype
Use new search service in getRecord(), performSearch(), getSimilarRecords()

* VuFind/Search/Solr/Params.php (getQuery): Return overrideQuery, if
  any.
* VuFind/Search/Solr/Results.php (getRecord,
  performSearch) (getSimilarRecords): Use new search
  service.  (createBackendParameters): New function. Create backend
  parameters based on Params.