Replace java.util.Hashtable with java.util.HashMap

Hashtable synchronizes every operation, which only costs time when a single thread uses it. This recipe replaces a Hashtable that a method creates and keeps in a local variable with a HashMap when program analysis shows that no other thread can reach it or a view of it such as its keySet(): neither is returned, stored in a field, passed to another method or constructor, or captured by a lambda that may run later or on another thread, by any path through the method, loops and catch blocks included. A table whose Hashtable-only methods (contains, elements, keys) are called, fields, and Kotlin and other non-Java source are left as they are. HashMap accepts null keys and values, so it accepts every input Hashtable did, and both leave the order of their keys unspecified.