Add ringbuffer sink

[eudoxos]

Ringbuffer sink keeps user-given number of most recent log messages in
memory and returns them upon request (using the ringbuffer_sink::last
method). This can be useful for e.g. remote debugging of a running app.

[gabime]

Thanks.
spdlog avoids 3rd party dependencies, so please use spdlog::details::circualr_q instead of boost's

[eudoxos]

Thanks.
spdlog avoids 3rd party dependencies, so please use [spdlog::details::circualr_q]

Done. I had to add circular_q::size() and circular_q::at(size_t) for random access without modifying the data.