Provide file size limitation for Block Disk Cache and Indexed Disk Cache

Attached please find a patch, that provides additional mode of operation to Block and Index Disk Caches, i.e. that instead of limiting the key count, you limit the content size. It's a proxy for limiting the file size, but it works quite good.

The patch has additional length, as I moved:
src/main/java/org/apache/commons/jcs/utils/struct/LRUMap.java
to:
src/main/java/org/apache/commons/jcs/utils/struct/AbstractLRUMap.java

and then - created LRUMap, that implements size-based policy.

Also, in a separate file I provided update for website docs.

I did some testing, also performance, and it looks like, it doesn't introduce noticeable difference.