Page cache that stores pages on disk. This class is XMLParamAware and needs the element cachedir
as a parameter. This element must contain the directory used to store the cached pages. This class
will also try to get a logger from the symbol table of the XMLParamProcessor with key "logger". If
this is not found, a SimpleLogger is used.
TODO: this class should be updated so that filenames are given sequentially and are then stored to
a HashMap with the original key. This avoids both too long filenames and possible (although unlikely)