Support WFS Paging at database level for MSSQL Driver

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Database Paging

WFS requests support a STARTINDEX parameter which allows results to be paged through in conjunction with MAXFEATURES.

A sample request would look as follows:

mapserv -nh "QUERY_STRING=map=test.map&service=WFS&version=2.0.0&request=GetFeature&typeName=MyLayer&outputFormat=geojson&sortBy=Property1%20ASC&startIndex=0&count=20"

The PostGIS driver supports paging at the database level. Other drivers fetch all the results and then MapServer carries out the paging in mapquery.c. Supporting drivers implement layer->vtable->LayerGetPaging.

Allowing WFS Paging at database level for the MSSQL Driver would greatly improve performance when paging through results.

MS SQL 2012+ can simulate paging in SQL with OFFSET and FETCH. A fallback for older versions (and for Azure hosted SQL Server) is RowNumber. The latter approach may be prereable as it is more universal.

Performance Issues

The lack of paging causes a huge overhead and performance hit when providing &startIndex=0&count=20. The database runs the full SQL query for the layer (with no paging) 3 times when running a WFS 2.0.0 GetFeature request:

1. In mapwfs.c with the msWFSRetrieveFeatures function (and then msLayerWhichShapes in mapquery.c (with no paging so the full query is run).

2. In mapwfs.c - msWFSComputeMatchingFeatures which again calls msWFSRetrieveFeatures and then LayerDefaultGetShapeCount in maplayer.c which runs the full query again.
   The MSSQL has the LayerGetShapeCount set to use the default rather than a database optimised SELECT COUNT(*) FROM.... This function does exist however in the driver as msMSSQL2008LayerGetNumFeatures and is set to: layer->vtable->LayerGetNumFeatures

Could this be used rather than LayerDefaultGetShapeCount?

3. Finally the query is reissued in msMSSQL2008LayerGetShape. msOGRWriteFromQuery in mapogroutput calls msLayerGetShape which then due to the following in the MSSQL driver causes another full query:

// variables are as follows
// resultindex = 0
// layerinfo->row_num = 6607 // all features in table

    if( resultindex < layerinfo->row_num) {
      /* re-issue the query */

SQL Profiler for a single GetFeature request below:

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Closed with 8ef65a2