SERVER-13489 move disk reading out of index scan ctor

(cherry picked from commit 5f3174ac46b169a90e1e17a7327a21824609abbf)
SERVER-13489 move possible disk access out of index cursor ctor

(cherry picked from commit ae2dc0a937cd66c249d0d98802fe8229e1a32df8)
SERVER-13500 prevent syncSourceFeedback segfault by not allowing NULL members to be added to _members map

(cherry picked from commit ba3823f2a7c08a022bebbe8accebba8893582e09)
Merge remote-tracking branch 'upstream/v2.6' into v2.6