Simplify debug level check

[taylanisikdemir]

What changed?

The recently added DebugOn() had some basic caching but benchmarks show that it doesn't matter much when debug level is not enabled. It helps save some cycles when log level is debug but is not worth the additional complexity (2 atomic operations etc.). Thanks @Groxx for pointing this out.

Benchmark results with level = debug

BenchmarkDebugOnCheckCached-12    	30902576	        38.90 ns/op	       0 B/op	       0 allocs/op
BenchmarkDebugOnCheckEachTime-12   	2618984	       		439.8 ns/op	     488 B/op	       4 allocs/op

Benchmark results with level = info

BenchmarkDebugOnCheckCached-12    	30820993	        38.28 ns/op	       0 B/op	       0 allocs/op
BenchmarkDebugOnCheckEachTime-12    30134168	        39.27 ns/op	       0 B/op	       0 allocs/op

So removing the cache and asking zap to check level each time.

[Groxx]

Thank ye :)

To stick it in writing too: I think the (marginal) added cost while debug-logging is completely fine because in debug-log-mode we really don't care about performance, and it's almost certainly dwarfed by whatever is being logged.

So all we really care about is prod behavior when not debug-logging, and that's essentially the same but much simpler now. Definitely seems worth it to avoid a pile of atomics.