SERVER-13960 fix access planning for OR with inexact child predicates

Includes a refactor of QueryPlannerAccess::processIndexScans(...) which makes this
change saner.

(cherry picked from commit bee249ac8907cc9de6b19ba87c3fcb074d84b1a3)
SERVER-13960 bug fixes for OR with inexact predicates

(cherry picked from commit 3242afb803a5cc523f16c2c63c3ee1dfc10a5671)