sentinel mode， lost "+switch-master" message, should client add watchdog thread

[chenshi5012]

I get some client and master-server in "vpc1"

redis-slave in "vpc2",

some sence, cause the network issue, failover happened, after some time the network issue resovle.

but the client lost the channel message from "+switch-master" .

happened some time under test envoirment.

can we make some cheduled active detection. to find out the master-failover. instead of the subscribe message form "+switch-master" 。

[sazzad16]

PRs are welcome.
Please allow configuration; i.e. enable/disable, schedule time, etc.