MEDIUM: stktable: split src-based key smp_fetch_sc functions

MEDIUM: stktable: split src-based key smp_fetch_sc functions

In this patch we split several sample fetch functions that are leveraged
by the "src-" fetches such as smp_fetch_sc_inc_gpc().

Indeed, for all of them, we add an intermediate helper function that takes
a stkctr pointer as parameter and performs the logic, leaving the lookup
part in the calling function. Before this patch existing functions were
doing the lookup + the fetch logic. Thanks to this patch it will become
easier to add generic converters taking lookup key as input.

List of targeted functions:
 - smp_fetch_sc_inc_gpc()
 - smp_fetch_sc_inc_gpc0()
 - smp_fetch_sc_inc_gpc1()
 - smp_fetch_sc_clr_gpc()
 - smp_fetch_sc_clr_gpc0()
 - smp_fetch_sc_clr_gpc1()
 - smp_fetch_sc_conn_cnt()
 - smp_fetch_sc_conn_rate()
 - smp_fetch_sc_updt_conn_cnt()
 - smp_fetch_sc_conn_curr()
 - smp_fetch_sc_glitch_cnt()
 - smp_fetch_sc_glitch_rate()
 - smp_fetch_sc_sess_cnt()
 - smp_fetch_sc_sess_rate()
 - smp_fetch_sc_http_req_cnt()
 - smp_fetch_sc_http_req_rate()
 - smp_fetch_sc_http_err_cnt()
 - smp_fetch_sc_http_err_rate()
 - smp_fetch_sc_http_fail_cnt()
 - smp_fetch_sc_http_fail_rate()
 - smp_fetch_sc_kbytes_in()
 - smp_fetch_sc_bytes_in_rate()
 - smp_fetch_kbytes_out()
 - smp_fetch_sc_gpc1_rate()
 - smp_fetch_sc_gpc0_rate()
 - smp_fetch_sc_gpc_rate()
 - smp_fetch_sc_get_gpc1()
 - smp_fetch_sc_get_gpc0()
 - smp_fetch_sc_get_gpc()
 - smp_fetch_sc_get_gpt0()
 - smp_fetch_sc_get_gpt()
 - smp_fetch_sc_bytes_out_rate()

Please note that this patch doesn't render any good using "git show" or
"git diff". For all the functions listed above, a new helper function was
defined right above it, with the same name without "_sc". These new
functions perform the fetch part, while the original ones (with "_sc")
now simply perform the lookup and then leverage the corresponding fetch
helper.