ext: arXiv Flask extension database search fix

* Adds an instance wide search parameter prefix to perform the database
  search.

* NOTE: for the search to be performed in the database, the arXiv field
  prefix for the corresponding instance config file should be added to
  invenio.cfg as ARXIV_SEARCH_PREFIX
  (e.g. ARXIV_SEARCH_PREFIX = u'035__a:oai:arXiv.org:').
see my previous comment https://github.com/inveniosoftware/invenio/pull/2106#discussion_r16328931