check performance with primitive ints instead of strings as doc id

Queries can be done in less than a millisecond even for hundreds of
thousands of documents.

pdb-keyword-db/build.gradle

@@ -2,9 +2,9 @@
 apply plugin: 'antlr'
 
 dependencies {
-	compile 'org.apache.commons:commons-collections4:4.1'
+	compile 'org.lucares:primitiveCollections:0.1.20170203201705'
 	runtime "org.antlr:antlr4:4.5.3"
-	antlr "org.antlr:antlr4:4.5.3" // use ANTLR version 4
+	antlr "org.antlr:antlr4:4.5.3"
 }
 
 sourceSets {