Make base address space abstract

[pbouill]

adding abstract classes for user manager, address space in order to allow for better implementation of iserver instantiation (allow iserver to use custom address space class)

[oroulet]

fine but I think these changes should be a few commits. one for each new abstract classes. Not 19 with some random rebases

[pbouill]

aha yeah -- sorry! will try to tidy up my branches before opening the next PR :)

[pbouill]

@oroulet -- do you want me to close this PR? These changes are included in #1835

[oroulet]

to be honest I am afraid of all these changes. A lot of chnages that makes code harder to follow and the use case is not really clear for me.
Also what are the impact on performamce on all the calls redirections?

[pbouill]

@oroulet intent was to allow for any user-implemented address space, and provide better type hinting for iserver/associated services... e.g. perhaps make an address space using redis or mongodb backend... would allow for much better data persistence + provide much better redundancy (and a more natural space for tag history to be stored). I can provide an implemented example shortly... are there specific speed/performance tests that you have in mind? e.g. just tag reads/writes?