Improve ZEL ImportHandler performance

Steps to Reproduce

repeated calls to resove EL variable suffer from a performance hit due to uncached unresolvable classes

	<div>
		<custom-attributes myvar="123"/>
		<div forEach="1,2,3">
			${myvar}
		</div>
	</div>

The code above leads to 3 caught ClassNotFoundExceptions in org.zkoss.zel.ImportHandler#findClass(String name)
https://github.com/zkoss/zk/blob/v8.6.0.1/zel/src/org/zkoss/zel/ImportHandler.java#L203-L207

The attached test scenario TestImportHandlerPerformance.java highlighting the difference between a cache hit and a cache miss at larger volumes.
At 100000 repetitions, resolving java.lang.Object takes ~<1ms, while repeated failing to resolve a variable 'test' results in significant slowdown (~600ms)

Possible Solution

This has been avoided in tomcat's EL implementation since 8.0.21

https://tomcat.apache.org/tomcat-8.0-doc/changelog.html#Tomcat_8.0.21_(markt)/Jasper

https://svn.apache.org/viewvc/tomcat/archive/tc8.0.x/tags/TOMCAT_8_0_21/java/javax/el/ImportHandler.java?revision=1848138&view=markup#l174

https://bz.apache.org/bugzilla/show_bug.cgi?id=57583

We should further evaluate if a later tomcat version (8.5 or 9 - all use EL 3.0 ) contains additional beneficial fixes and improvements to merge in to ZK's EL adaptation.