SERVER-15500 SERVER-15515 system.users tests

This re-applies (with fixes) the following commits:
    0a98d654a7054d252dd921d27a5649f7177c8723.
    9e58312f0363b8164b59dc2541b733d7417a9986.

Closes #815

Signed-off-by: Benety Goh <benety@mongodb.com>
SERVER-15491 catch Socket exceptions when authenticating SyncSourceFeedback connections
SERVER-15528 Refactor DistinctScan to do less in each work() call

(cherry picked from commit e0f47214d06738f14d0d0d731235a9f665a6b81f)
SERVER-14287 do not run bg index builds that aren't interruptable
SERVER-14184 Remove unused S2 macros that conflict on OS X Yosemite

(cherry picked from commit 759b6e8cabfb745b712b5ffd0748561129c3b421)
SERVER-15673 Disable SSLv3 ciphers (CVE-2014-3566 "POODLE")

(cherry picked from commit bdee10aed8691579d17300a1235edce8742ef6dd)
Merge branch 'v2.6' of github.com:mongodb/mongo into v2.6