Provide annotations to document thread safety

Since already have a package called org.apache.commons.lang3.concurrent, I propose we place these annottaions in org.apache.commons.lang3.concurrent.annotation.

The annotations are:

• org.apache.commons.lang3.concurrent.annotation.GuardedBy
• org.apache.commons.lang3.concurrent.annotation.Immutable
• org.apache.commons.lang3.concurrent.annotation.NotThreadSafe
• org.apache.commons.lang3.concurrent.annotation.ThreadSafe

The inspiration for these annotations are:

• JSR 305: https://jcp.org/en/jsr/detail?id=305
• JCIP: http://jcip.net/annotations/doc/index.html
• com.google.code.findbugs : jsr305 : 3.0.1: https://search.maven.org/#artifactdetails%7Ccom.google.code.findbugs%7Cjsr305%7C3.0.1%7Cjar

The JCIP code is released under the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.5)

The NOTICE file will be updated to reflect this.

Annotation retention will be at the CLASS level which does NOT create a runtime dependency on Commons Lang.