fix tracing enabled when log level is LogLevelNone

[C-Pro]

I noticed a significant CPU time spent in traceInPush even though my log level is LogLevelNone.

The core issue is that log level constants are not ordered in a consistent way (neither high to low, nor low to high):

LogLevelNone LogLevel = iota	// 0 - is no logging
LogLevelTrace			// 1 - max verbosity
LogLevelDebug			// 2 - high verbosity

Because of this, check level >= c.config.LogLevel does not work as expected in logLevelEnabled and for clients with LogLevel not specified (zero value is LogLevelNone) logLevelEnabled always returns true.
Fixing the order of constants would be a proper fix, but can possibly break some users code depending on numerical values.
Instead I add explicit check for LogLevelNone in logLevelEnabled. This should be backwards compatible, unless users actually expect LogLevelNone to be equal to LogLevelTrace.

[FZambia]

Oh, unfortunate... Thanks a lot!