Optimize repository search by changing our lookup strategy

Previous to this change, repositories were looked up unfiltered in six different queries, and then filtered using the permissions model, which issued a query per repository found, making search incredibly slow. Instead, we now lookup a chunk of repositories unfiltered and then filter them via a single query to the database. By layering the filtering on top of the lookup, each as queries, we can minimize the number of queries necessary, without (at the same time) using a super expensive join.

Other changes:
- Remove the 5 page pre-lookup on V1 search and simply return that there is one more page available, until there isn't. While technically not correct, it is much more efficient, and no one should be using pagination with V1 search anyway.
- Remove the lookup for repos without entries in the RAC table. Instead, we now add a new RAC entry when the repository is created for *the day before*, with count 0, so that it is immediately searchable
- Remove lookup of results with a matching namespace; these aren't very relevant anyway, and it overly complicates sorting

data/users/externalldap.py

@@ -1,10 +1,10 @@
 import ldap
 import logging
 import os
-import itertools
 
 from collections import namedtuple
 from data.users.federated import FederatedUsers, UserInformation
+from util.itertoolrecipes import take
 
 logger = logging.getLogger(__name__)
 
@@ -57,10 +57,6 @@ class LDAPConnection(object):
   def __exit__(self, exc_type, value, tb):
     self._conn.unbind_s()
 
-def _take(n, iterable):
-  "Return first n items of the iterable as a list"
-  return list(itertools.islice(iterable, n))
-
 
 class LDAPUsers(FederatedUsers):
   _LDAPResult = namedtuple('LDAPResult', ['dn', 'attrs'])
@@ -154,7 +150,7 @@ class LDAPUsers(FederatedUsers):
         return (None, err_msg)
 
       logger.debug('Found matching pairs: %s', pairs)
-      results = [LDAPUsers._LDAPResult(*pair) for pair in _take(limit, pairs)]
+      results = [LDAPUsers._LDAPResult(*pair) for pair in take(limit, pairs)]
 
       # Filter out pairs without DNs. Some LDAP impls will return such pairs.
       with_dns = [result for result in results if result.dn]