rtpengine: Reduce lock contention during ping timer

Hi,

I've identified a lock contention issue in the rtpengine module's ping
timer that can significantly impact call processing under certain
conditions.

**Problem:**
The `rtpengine_ping_check_timer()` function, triggered periodically by
`ping_interval`, holds locks on `rtpp_set_list` for the entire duration of
pinging all rtpengine nodes.

Normally, this works find, but when rtpengine instances become unreachable,
the lock is held for an long time. For example:

- 2 unreachable rtpengine nodes
- 1000ms command timeout
- 5 retries per node
- locks held for ~10 seconds

**Proposed Solution:**
Split the ping operation into three phases:
1. **Phase 1 (with lock):** Create a snapshot of node pointers
2. **Phase 2 (without lock):** Perform actual ping operations on the
snapshot
3. **Phase 3 (with lock):** update node state

Since rtpengine nodes are never freed during runtime, the pointers in the
snapshot remain valid throughout the ping cycle so lock is not required.

I have implemented and tested this change in my deployment. Should I submit
a pull request, or is there a preferred alternative approach?

regards,
Rajneesh

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