Consider refactoring various code that makes descriptors persistent

"How much does the new function in PersistenceUtils differ from the similar functions:

onionperf/OnionPerfDownloader.java:  public void cleanUpRsyncDirectory() throws ConfigurationException {
exitlists/ExitListDownloader.java:  public void cleanUpRsyncDirectory() {
bridgedescs/SanitizedBridgesWriter.java:  public void cleanUpRsyncDirectory() throws ConfigurationException {
relaydescs/ArchiveWriter.java:  public void cleanUpRsyncDirectory() {

Currently webstats does seem to be the only module that uses the shared function. The relaydescs module may need something more complex to handle persisting microdescriptors for longer, but the other modules could maybe just switch? This could be a new ticket."

Needs discussion first.