DEBUG_PTR does not work with base 7.0.7

[simon-ess]

The flag DEBUG_PTR was added in order to better track shared pointer usage. However, it is not compatible with a few changes to EPICS base since then:

• https://github.com/epics-base/pvAccessCPP/commit/161eac3a383583b0941cc4f1ec91c1c16a9887ef - adds ->expired() to weak_ptr
• https://github.com/epics-base/pvAccessCPP/commit/32c51c8 - adds write to an ostream that is not resolve correctly (as near as I can tell, due to another possible match in reftrack.h).

Proposed fix will be added in a merge request.