replace EhCache with a custom implementation

The cache must remove/evict writers after a few seconds, but EhCache
only evicts entries when a new entry is added. That is not acceptable
for us, because that would leave lots of files open and we would need
a second mechanism to close them.
Therefore I write a simple wrapper for a ConcurrentHashMap that evicts
entries after timeToLive+5s.

performanceDb/build.gradle

@@ -5,8 +5,6 @@ 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'
 	compile 'org.apache.logging.log4j:log4j-core:2.10.0'