Added connection pool for postgres
Change the expiration timeout to one minute.
WMS provider fixes for multi-threading

This is still far from working nicely or being finished:
- cloned provider does not get capabilities copied over, so they are requested again (for each redraw!)
- legend graphic download is disabled for now, was causing crashes while resizing window / manipulating docks
- tiled requests probably do not work
WFS: added featureSource() - now it is possible to do rendering
WCS: quick and dirty way to get it working
GRASS raster: remove superfluous QProcess instances
GRASS provider updated to support featureSource()
GPX: added support for featureSource()
delimitedtext: move methods specific to feature iterator from provider to FI
Fix issue with combination of edit buffer + vector join
delimitedtext: add support for featureSource()
Fixed point displacement renderer (and made it faster)
Make insertVertex/moveVertex/deleteVertex work without cache
Fixed bug in the processing toolbox qgis:intersection tool, preventing
attempts at writing the wrong geometry type to a shapefile.
Moved WMS capabilities, URI parsing out of provider class.

With this change, all WMS capabilities are kept within one class.
When the provider is cloned, the existing capabilities object is
passed to new provider, thus not requiring new provider download
capabilities again.

Data items and GUI dialog do not have to create a dummy provider
just in order to get capabilities.
Resolved handling of WMS requests in worker threads
WCS provider: use a download handler (like WMS)
Allow Qt to search for Qt plugins (e.g. sqldrivers) in our plugin directory

Without this it is necessary to explicitly provide QT_PLUGIN_PATH environment
variable for Qt plugins we build (e.g. Oracle DB sql driver)
Oracle provider: added featureSource() support
Server adjustments (allow usage from build dir, new API)