MINOR: stats: prepare stats-file support for values other than FN_COUNTER

MINOR: stats: prepare stats-file support for values other than FN_COUNTER

Currently, only FN_COUNTER are dumped and preloaded via a stats-file.
Thus in several places we relied on the assumption that only FN_COUNTER
are valid in stats-file context.

New stats types will soon be implemented as they are also eligilible to
statistics reloading on process startup. Thus, prepare stats-file
functions to remove any FN_COUNTER restriction.

As one of this change, generate_stat_tree() now uses stcol_is_generic()
for stats name tree indexing before stats-file parsing.

Also related to stats-file parsing, individual counter preloading step
as been extracted from line parsing in a dedicated new function
load_ctr(). This will allow to extend it to support multiple mechanism
of counter preloading depending on the stats type.