Trigger the election immediately when doing a manual failover

[enjoy-binbin]

Currently when a manual failover is triggeded, we will set a
CLUSTER_TODO_HANDLE_FAILOVER to start the election as soon as
possible in the next beforeSleep. But in fact, we won't delay
the election in manual failover, waitting for the next beforeSleep
to kick in will delay the election a some milliseconds.

We can trigger the election immediately in this case in the
same function call, without waitting for beforeSleep, which
can save us some milliseconds.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.64%. Comparing base (bb57dfe) to head (d926d8e).

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable    #1081      +/-   ##
============================================
+ Coverage     70.61%   70.64%   +0.03%     
============================================
  Files           114      114              
  Lines         61694    61695       +1     
============================================
+ Hits          43564    43587      +23     
+ Misses        18130    18108      -22     

Files with missing lines	Coverage Δ
src/cluster_legacy.c	86.29% <100.00%> (+0.13%)	⬆️

... and 12 files with indirect coverage changes