[stats] Rewrite stat management to use single threaded event loop

[max-hoffman]

Reference statspro/doc.go for more detailed overview. Replaced stats management with an event loop and shared bucket cache. Rudimentary GC and branch syncing seems like a must for thousands of branches.

Branch add/delete seems to work as I'd expect under high concurrency. The GC and branch syncing concurrency is a bit sloppy right now. It doesn't deadlock, but thousands of concurrent adds a second while running GC/branch sync every <10ms doesn't true up the way I would want.

Early perf testing seems to have little/no impact on TPC-C when the job ticker is 100-500ms. Every few minutes during TPC-C stats throws spurious errors that appear to self-resolve. It doesn't exactly clutter output, but I think users would be happier if I figured how out how to reduce those errors.

fixes: #8844

[coffeegoddd]

@max-hoffman DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
4e12e77	ok	5937457

version	total_tests
4e12e77	5937457

correctness_percentage
100.0

[coffeegoddd]

@max-hoffman DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
10f8bc3	ok	5937457

version	total_tests
10f8bc3	5937457

correctness_percentage
100.0

[coffeegoddd]

@max-hoffman DOLT

comparing_percentages
100.000000 to 100.000000

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10e67a7	ok	5937457

version	total_tests
10e67a7	5937457

correctness_percentage
100.0

[coffeegoddd]

@max-hoffman DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
58879d6	ok	5937457

version	total_tests
58879d6	5937457

correctness_percentage
100.0

[coffeegoddd]

@max-hoffman DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
0e4c2e5	ok	5937457

version	total_tests
0e4c2e5	5937457

correctness_percentage
100.0

[coffeegoddd]

@max-hoffman DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
bb69370	ok	5937457

version	total_tests
bb69370	5937457

correctness_percentage
100.0

[coffeegoddd]

@coffeegoddd DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
687f497	ok	5937457

version	total_tests
687f497	5937457

correctness_percentage
100.0

[coffeegoddd]

@max-hoffman DOLT

comparing_percentages
100.000000 to 100.000000

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7864cf1	ok	5937457

version	total_tests
7864cf1	5937457

correctness_percentage
100.0

[coffeegoddd]

@max-hoffman DOLT

comparing_percentages
100.000000 to 100.000000

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6c8a0db	ok	5937457

version	total_tests
6c8a0db	5937457

correctness_percentage
100.0

[coffeegoddd]

@max-hoffman DOLT

comparing_percentages
100.000000 to 100.000000

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9e15822	ok	5937457

version	total_tests
9e15822	5937457

correctness_percentage
100.0