Make IntelliJ's inspection "ThreadLocal field not declared static final" a error

[leventov]

This is often a bug or a code smell that is always possible to refactor into something better: see #8884, #8886, #8887.

[leventov]

The same effect could be attained via Checkstyle:

<!-- Ensure all ThreadLocal fields are private -->
<!-- Requires https://github.com/sevntu-checkstyle/sevntu.checkstyle -->
<module name="AvoidModifiersForTypesCheck">
  <property name="forbiddenClassesRegexpProtected" value="ThreadLocal"/>
  <property name="forbiddenClassesRegexpPublic" value="ThreadLocal"/>
  <property name="forbiddenClassesRegexpPackagePrivate" value="ThreadLocal"/>
</module>

<!-- Prohibit any ThreadLocal field which is not private static final -->
<module name="Regexp">
  <property name="id" value="nonStaticThreadLocal"/>
  <property name="format"
    value="^\s*private\s+(ThreadLocal|static\s+ThreadLocal|final\s+ThreadLocal)"/>
  <property name="illegalPattern" value="true"/>
  <property name="message" value="Non-static final ThreadLocal"/>
</module>