lake: make rangeWrapper filters safe for concurrent use #2969
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Because of concurrency in the ZNG scanner, separate filters returned by
a zbuf.Filter's methods must be safe for concurrent use in separate
goroutines. Filters returned by lake.rangeWrapper.AsFilter do not have
this property because they share the single expr.ValueCompareFn in
rangeWrapper.compare. Fix this by creating a new expr.ValueCompareFn
for each filter returned.
Closes #2960.