time repr - Reuse epoch object instead if making a new object.

[vEpiphyte]

according to my timeit tests, this is about 10% faster than constructing the new object each time

[codecov]

Codecov Report

Merging #1919 into master will decrease coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1919      +/-   ##
==========================================
- Coverage   96.69%   96.61%   -0.08%     
==========================================
  Files         139      139              
  Lines       26501    26501              
==========================================
- Hits        25624    25603      -21     
- Misses        877      898      +21     

Flag	Coverage Δ
#linux	96.61% <100.00%> (-0.08%)	⬇️
#linux_replay	70.92% <100.00%> (-0.04%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
synapse/lib/time.py	98.79% <100.00%> (ø)
synapse/lib/spawn.py	89.10% <0.00%> (-6.42%)	⬇️
synapse/common.py	97.16% <0.00%> (-0.32%)	⬇️

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