Group-access roles for SQL functions (draft)

Group-access roles for SQL functions (draft)

This is attempt for fine-grained access rights for all
Skytools SQL schemas.  As it still needs review,
the rights are not activated by default, instead
following sql files are generated:

  newgrants_<schema>.sql - applies new rights, drop old public access

  oldgrants_<schema>.sql - restores old rights - public execute
  		           privilege to all functions

Only thing that is active by default is creation of new
groups in upgrade functions.

New access roles:

pgq_reader
    Can consume queues (source-side)

pgq_writer
    Can write into queues (source-side / dest-side)
    Can use pgq_node/pgq_ext schema as regular
    consumer (dest-side)

pgq_admin
    Admin operations on queues, required for CascadedWorker on dest-side.
    Member of pgq_reader and pgq_writer.

londiste_reader
    Member of pgq_reader, needs additional read access to tables.
    (source-side)

londiste_writer
    Member of pgq_admin, needs additional write access to tables.
    (dest-side)