simplify caching in TagsToFile

- PdbFiles no longer require dates to be monotonically
  increasing. Therefore TagsToFile does not have to ensure
  this. => We only have one file per Tags.
- Use EhCache instead of HashMap.

performanceDb/build.gradle

@@ -5,6 +5,7 @@ dependencies {
 	compile project(':file-utils')
 	compile 'com.fasterxml.jackson.core:jackson-databind:2.9.7'
 	compile 'org.apache.commons:commons-collections4:4.2'
+	compile 'org.ehcache:ehcache:3.6.1'
 	
 	
 	compile 'org.apache.logging.log4j:log4j-api:2.10.0'