BUG/MEDIUM: stick-tables: fix breakage in table converters

BUG/MEDIUM: stick-tables: fix breakage in table converters

Baptiste reported that the table_conn_rate() converter would always
return zero in 1.6.5. In fact, commit bc8c404 ("MAJOR: stick-tables:
use sample types in place of dedicated types") broke all stick-table
converters because smp_to_stkey() now returns a pointer to the sample
instead of holding a copy of the key, and the converters used to
reinitialize the sample prior to performing the lookup. Only
"in_table()" continued to work.

The construct is still fragile, so some comments were added to a few
function to clarify their impacts. It's also worth noting that there
is no point anymore in forcing these converters to take a string on
input, but that will be changed in another commit.

The bug was introduced in 1.6-dev4, this fix must be backported to 1.6.