Explicitly sync when slab replay log gets too big

[jnwatson]

Also keep stats on replay log size and commit time.

[codecov]

Codecov Report

Merging #1838 into master will increase coverage by 4.20%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1838      +/-   ##
==========================================
+ Coverage   86.95%   91.15%   +4.20%     
==========================================
  Files         140      140              
  Lines       27433    27441       +8     
==========================================
+ Hits        23855    25015    +1160     
+ Misses       3578     2426    -1152     

Flag	Coverage Δ
#linux	91.15% <100.00%> (+4.20%)	⬆️
#linux_replay	57.38% <90.90%> (-9.11%)	⬇️

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Impacted Files	Coverage Δ
synapse/cortex.py	92.80% <100.00%> (-2.85%)	⬇️
synapse/lib/lmdbslab.py	98.47% <100.00%> (-0.61%)	⬇️
synapse/tools/guid.py	0.00% <0.00%> (-100.00%)	⬇️
synapse/tools/autodoc.py	0.00% <0.00%> (-96.06%)	⬇️
synapse/tools/pullfile.py	0.00% <0.00%> (-91.67%)	⬇️
synapse/cmds/hive.py	18.84% <0.00%> (-81.16%)	⬇️
synapse/lib/stormwhois.py	26.66% <0.00%> (-73.34%)	⬇️
synapse/cmds/cortex.py	39.58% <0.00%> (-59.03%)	⬇️
synapse/models/dns.py	49.15% <0.00%> (-50.85%)	⬇️
synapse/models/telco.py	53.40% <0.00%> (-46.60%)	⬇️
... and 46 more

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