Fix leap day agenda bug

[jnwatson]

Handle next time of appointment when today is leap day.

Symptom was: ValueError: day is out of range for month when the appointment crossed years.

[codecov]

Codecov Report

Merging #1550 into master will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1550      +/-   ##
==========================================
- Coverage   94.40%   94.37%   -0.04%     
==========================================
  Files         133      133              
  Lines       23463    23466       +3     
==========================================
- Hits        22150    22145       -5     
- Misses       1313     1321       +8     

Flag	Coverage Δ
#linux	94.37% <100.00%> (+<0.01%)	⬆️
#osx	?

Impacted Files	Coverage Δ
synapse/lib/platforms/darwin.py	0.00% <0.00%> (-100.00%)	⬇️
synapse/lib/thisplat.py	58.33% <0.00%> (-8.34%)	⬇️
synapse/lib/spawn.py	92.21% <0.00%> (-1.17%)	⬇️
synapse/lib/link.py	98.66% <0.00%> (-0.67%)	⬇️
synapse/daemon.py	88.98% <0.00%> (-0.30%)	⬇️
synapse/cortex.py	92.20% <0.00%> (-0.06%)	⬇️
synapse/lib/lmdbslab.py	98.10% <0.00%> (+0.37%)	⬆️

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[vEpiphyte]

This was cherry-picked directly in the 01x branch.