[Fix] Use pure ev_timer for TCP session timeouts instead of rspamd_io_ev

[Fix] Use pure ev_timer for TCP session timeouts instead of rspamd_io_ev

Replace rspamd_io_ev with pure ev_timer for TCP session timeouts.
rspamd_io_ev is a wrapper for combined IO+timer watchers and creates
unnecessary overhead when used for pure timers:

- Changed session->timer_ev from rspamd_io_ev to ev_timer
- Simplified callback signature to native libev callback
- Use ev_timer_init/ev_timer_start/ev_timer_stop directly
- Removed unnecessary wrapper functions and struct fields

This eliminates wasted memory from dummy ev_io structs and clarifies
the separation between IO watchers and timer-only watchers.