BUG/MEDIUM: stktable: fix missing lock on some table converters

BUG/MEDIUM: stktable: fix missing lock on some table converters

In 819fc6f563
("MEDIUM: threads/stick-tables: handle multithreads on stick tables"),
sample fetch and action functions were properly guarded with stksess
read/write locks for read and write operations respectively, but the
sample_conv_table functions leveraged by "table_*" converters were
overlooked.

This bug was not known to cause issues in existing deployments yet (at
least it was not reported), but due to its nature it can theorically lead
to inconsistent values being reported by "table_*" converters if the value
is being updated by another thread in parallel.

It should be backported to all stable versions.

[ada: for versions < 3.0, glitch_cnt and glitch_rate samples should be
 ignored as they first appeared in 3.0]