Modules: preserved per-entry TTL on shared dict incr() calls.

Modules: preserved per-entry TTL on shared dict incr() calls.

Previously, incr() without an explicit timeout argument always
reset the entry expiry to the directive default, discarding any
per-entry timeout set by a prior add(), set(), or incr() call.

This aligns the behavior with Redis INCR and OpenResty
ngx.shared.DICT:incr() where value mutation does not touch the
existing TTL.  An explicit timeout argument still updates it.