Cleanup and restructure parallel solver helpers

The following discussion from !1839 (closed) should be addressed:

@timok started a discussion: (+2 comments)

@blattms Do I see it right that all solver use the same parallel setup as the AMG? So amgtraits.hh and amgparallelhelpers.hh are not specific to AMG? Then we could just rename the headers and AMGTraits->ParallelSolverTraits so it's less confusing.

Move amgparallelhelpers.hh to paralellhelpers.hh or similar -> fixed in !1848 (merged)
AmgTraits -> LinearSolverTraits? -> fixed in !1848 (merged)
LinearAlgebraTraits?
Merge linear/vectorexchange.hh and parallel/vertexhandles.hh -> fixed in !1848 (merged)

Edited Jan 30, 2020 by Timo Koch