BUG/MINOR threads: Use get_(local|gm)time instead of (local|gm)time

BUG/MINOR threads: Use get_(local|gm)time instead of (local|gm)time

Using localtime / gmtime is not thread-safe, whereas the `get_*` wrappers are.

Found using GitHub's CodeQL scan.

The use in sample_conv_ltime() can be traced back to at least
fac9ccfb705702f211f99e67d5f5d5129002086a (first appearing in 1.6-dev3), so all
supported branches with thread support are affected.

(cherry picked from commit 1f269c12dc31bb63db31559cb44c187ab91abb64)
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit 9da4b1fc83fe083c8f194c61402ba4ffeb5b330c)
Signed-off-by: Willy Tarreau <w@1wt.eu>