UimaContext_ImplBase code to generate uniqueIdentifier may not be unique

Code in the construction of UimaContext_ImplBase calls UID() to get a unique id. One use is as a way to allow (for scaled-out apps) unique identification of otherwise similar MBeans, using different mbean names. MBean names cannot have colons or dashes. This code removes those from the string value returned by UID(). This means that 2 different UIDs that differ only in having some part have a minus sign, could be confused with another not having this. A small test case shows this can arise if > 32768 UIMA Contexts are created within one millisecond. While extremely unlikely, this is easily avoided, for example, by substituting _ for the characters, rather than deleting them.