Parse and validate request processing rules only once

[h-arlt]

Request processing rules are parsed and validated for each invocation of ReportGeneratorConfiguration#getRequestProcessingRules(), or more specifically, for each data parser thread.

This may be improved by shifting the parse-and-valdidate work into the ReportGeneratorConfiguration's constructor such that it is done only once. However, since request processing rules are composed of stateful filters, method getRequestProcessingRules still need to return copies of the parsed rules.