Elevation feature is optional

This commit makes all the changes related to the elevation feature
optional, controlled by the boolean CMake parameter OSRM_HAS_ELEVATION.
If this parameter is not specified at configure time, then the elevation
feature is not enabled.
Fixing default value of elevation parameter

If the elevation feature is enabled, the elevation parameter in the NodeInfo
data structure for nodes that do not provide elevation information is
initialized to INT_MAX.
Adding support for the elevation feature for PBF

This commit introduces support for the elevation feature in the PBF
extractor class.
Elevation support in GPX descriptor

This commit introduces support for the output of elevation data in the
GPX format descriptor.
Elevation feature in SharedDataFacade

This commit introduces support for the elevation feature when using
shared memory, through the SharedDataFacade