[FIX] OPW 577963: ir_attachment: speed up ir.attachment search for large databases

[FIX] OPW 577963: ir_attachment: speed up ir.attachment search for large databases

  Frequents calls to list.remove() were being a bottleneck for large
  document lists (100k+). Using a set make remove() calls much faster.

  Also turned a read() into a pure SQL command, as its high-level
  features were unnecessary (security checks performed before and
  after it anyways) and it was 50% slower than the direct SQL version.

  This commit has a corresponding addons patch in order to use a set
  instead of a list in the document module as well.

bzr revid: xal@openerp.com-20120821143926-ta75gd98qi30wbp9