Optimise storage/calculation of geometry bounding boxes

[nyalldawson]

This commit delivers a few optimisations to geometry classes:

1. Removes the storage of an unnecessary QgsRectangle bounding box from QgsPointV2 (gives significant decrease in size of object: 80->48 bytes). This also has follow one benefits to all other geometry classes which storage nodes as QgsPointV2.
2. Implements a more efficient calculation method for the bounding box of linestrings.

Additionally, this commit moves the bounding box invalidation to a virtual QgsAbstractGeometryV2::clearCache() method, so that other non-bounding box caches can also be cleared when the geometry is modified. @jef-n #2817 will benefit from this (and also the smaller QgsPointV2 objects)

[jef-n]

then the local mCS in #2817 should probably disappear from QgsAbstractGeometryV2 to avoid it in QgsPointV2. But what should the reference returned by QgsPointV2::coordinateSequence() point at in that case?

[nyalldawson]

@jef-n maybe don't return a reference. QList is implicitly shared anyway, so returning a copy should be inexpensive. And then QgsPointV2 can just create a new sequence with a copy of the point instead.