gstinfo: Improve usage of libdw for backtraces

gstinfo: Improve usage of libdw for backtraces

When getting backtraces, we were always creating a new Dwfl context and then
discarding it. The problem with that is that it resulted in having to re-scan a
lot of information for every single backtrace.

In order to fix that issue, use a global on-demand Dwfl context and use it with
a lock.

Furthermore, we were scanning the mappings of the
process (dwfl_linux_proc_report) for *every single step* in the backtrace, and
that function is horrendously expensive (does sscanf on /proc/PID/maps
...). While there is a possibility that new mappings might be available (new
plugins being loaded for example), we can limit ourselves to just do it once per
backtrace.

These two modifications speed up the elements_leaks unit test (which traces all
pads with full backtraces) by a factor of 6.

Partially fixes #567

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/504>