Add pqxx magic to allow use of arrays

This magic allows the use of bigint[] arrays. See https://github.com/zerebubuth/openstreetmap-cgimap/issues/58#issuecomment-24663264
Add a new function for inserting results into a list in the readonly backend

This also allows us to replace insert_results_of with a call to
insert_results

Eventually we want to rewrite everything to use insert_results, but
this allows it to be done incrementally.
Convert select elements from lists to prepared statements for readonly
Add template<> magic to readonly for std::set too
Use prepared statements in readonly for selecting objects from lists of other objects
Use a prepared statement for readonly nodes in bbox

Fixes #68 for the readonly backend
Merge pull request #78 from pnorman/readonly_arrays

Convert readonly backend to use prepared statements and arrays