Add database caching to more functions

This patch dramatically reduces the number of database queries,
especially on the first page of the tracker with projects that have
custom fields.

• Added database cache to bug_get_bugnote_stats_array(),
  custom_field_cache_array_rows() and filter_db_get_project_current()
• Added database negative result cache to category_cache_array_rows()
• Fixed database cache in file_bug_attachment_count_cache()
• Added cache of all relationships

Fixes 0036889, PR https://github.com/mantisbt/mantisbt/pull/2189

Signed-off-by: Damien Regad dregad@mantisbt.org