Generic cache management algorithm

Generic cache management algorithm
(*) Transcoder class implements translation between alien ids (ids referring to another peer's DB) into the local DB
(*) XrefTable class implements DB access to bilateral n-to-n relationsships like slice_node
(*) Cache basically gets object lists from the peer, and calls update_table sequentially

A lot of stuff was removed from Peer and RefreshPeer, as well as Slices and was rewritten here

The classes subject to this now need to define extra attributes:
(*) class_id : e.g. 'node_id'
(*) class_key : e.g. 'hostname'
(*) foreign_fields : the list of stuff we copy verbatim from foreign objects
(*) foreign_xrefs : specify what cross-references should be solved and how, see Slices for an example

yet to do:
(*) improve intermediate objects allocation (transcoders and xreftables are created on the fly)
(*) use for importing keys, persons and slice attributes.