Eliminate a Query deep copy when constructing RLMArrayTableView

Query doesn't support move-assignment and obj-c++ requires that C++ data
members of obj-c classes be default-constructed, so heap-allocate queries to
eliminate the copy assignment.

Makes queries bound by the Query construction/destruction time roughly twice as fast.
Merge pull request #944 from realm/tg-eliminate-query-copy

Eliminate a Query deep copy when constructing RLMArrayTableView