Fix filter_policy object leak in NodeStore backends
Replace const Type& with Type const& for common types.

* std::string
* RippleAccount
* Account
* Currency
* uint256
* STAmount
* Json::Value
Make NetworkOPs::isFull() thread-safe
Improvements to aged_containers (RIPD-363)

- Added unit tests for element erase
- Added unit tests for range erase
- Added unit tests for touch
- Added unit tests for iterators and reverse_iterators
- Un-inlined operator== for unordered containers
- Fixed minor problems with ordered_container erase()
- Made ordered_container...
  - erase (reverse_iterator pos) not compile
  - erase (reverse_iterator first, reverse_iterator last) not compile
  - touch (reverse iterator pos) not compile
- Verified that ordered container...
  - insert() already rejects reverse_iterator
  - emplace_hint() already rejects reverse_iterator
- Made set/multiset iterators const

Regarding the set/multiset iterators, see section 1.5 of
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2009/n2913.pdf
as pointed out by Vinnie.