Sentinel: improve handling of known Sentinel instances.
Sentinel: improve handling of known Sentinel instances.

1. Bug #3035 is fixed (NULL pointer access). This was happening with the
   folling set of conditions:

* For some reason one of the Sentinels, let's call it Sentinel_A, changed ID (reconfigured from scratch), but is as the same address at which it used to be.

* Sentinel_A performs a failover and/or has a newer configuration compared to another Sentinel, that we call, Sentinel_B.

* Sentinel_B receives an HELLO message from Sentinel_A, where the address and/or ID is mismatched, but it is reporting a newer configuration for the master they are both monitoring.

2. Sentinels now must have an ID otherwise they are not loaded nor persisted in the configuration. This allows to have conflicting Sentinels with the same address since now the master->sentinels dictionary is indexed by Sentinel ID.

3. The code now detects if a Sentinel is annoucing itself with an IP/port pair already busy (of another Sentinel). The old Sentinel that had the same port/pair is set as having port 0, that means, the address is invalid. We may discover the right address later via HELLO messages.

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