SNMP: Replace mib_tree_entry struct with MEMPROXY class

[yadij]

No description provided.

[yadij]

Steps 1 (use MEMPROXY for new/delete) and 2 (C++11 initialization and constructor)
of a long refactor to replace the MIB tree with STL container(s) and RAII memory management.

[rousskov]

Several other change requests also avoid in-class initialization of members that must be initialized via class constructor (assuming that they must be). In this and other PRs, let's give compiler a chance to warn us if we forget to initialize such members while refactoring object construction code.

[kinkie]

Is there any specific reasons for not including parsefunction and instancefunction in the constructor?

[yadij]

Simplicity for now. Later refactor steps will change them.

[kinkie]

What does done look like with the later refactor steps?

[rousskov]

What does done look like with the later refactor steps?

I suspect there will be two mib_tree_entry constructors, one setting (constant) parse/instance functions and one prohibiting them. Or two C++ class types to differentiate the two use cases. Or perhaps a lot more classes, with those functions converted to class methods. Delaying that non-trivial refactoring may be the right decision, but I have not studied all the necessary details to be sure.