InputStreamImpl should hold a shared_ptr of the BlockReader

The remote block reader relies on undefined behavior in how it uses enable_shared_from_this.

http://en.cppreference.com/w/cpp/memory/enable_shared_from_this

The spec states a shared_ptr to an object inheriting from enable_shared_from_this must be live before calling make_shared_from_this.
Calling make_shared_from_this without an existing shared_ptr is undefined behavior and causes deterministic crashes when the code is built with GCC.

example:
class foo : public enable_shared_from_this

;

safe:
auto ptr1 = std::make_shared<foo>();
auto ptr2 = foo->make_shared_from_this();

broken:
foo *ptr = new foo();
auto ptr2 = foo->make_shared_from_this(); //no existing live shared_ptr

In order to fix the input stream should call std::make_shared and hang onto a shared_ptr to the block reader. The block reader will then be free to call make_shared_from this as much as it wants without issue.