Address Error Prone warnings

[armiol]

As of this moment there is a couple of warnings to address:

/Users/foobar/development/Spine/web/web/src/main/java/io/spine/web/parser/Base64MessageParser.java:77: warning: [ImmutableEnumChecker] enums should be immutable: 'LogSingleton' has field 'value' of type 'org.slf4j.Logger', the declaration of type 'org.slf4j.Logger' is not annotated with @com.google.errorprone.annotations.Immutable
        private final Logger value = LoggerFactory.getLogger(Base64MessageParser.class);
                             ^
    (see https://errorprone.info/bugpattern/ImmutableEnumChecker)
/Users/foobar/development/Spine/web/web/src/main/java/io/spine/web/parser/JsonMessageParser.java:130: warning: [ImmutableEnumChecker] enums should be immutable: 'LogSingleton' has field 'value' of type 'org.slf4j.Logger', the declaration of type 'org.slf4j.Logger' is not annotated with @com.google.errorprone.annotations.Immutable
        private final Logger value = LoggerFactory.getLogger(JsonMessageParser.class);
                             ^
    (see https://errorprone.info/bugpattern/ImmutableEnumChecker)
2 warnings

and

Task :firebase-web:compileTestJava
/Users/foobar/development/Spine/web/firebase-web/src/test/java/io/spine/web/firebase/given/FirebaseQueryMediatorTestEnv.java:53: warning: [BadImport] Importing nested classes/static methods/static fields with commonly-used names can make code harder to read, because it may not be clear from the context exactly which type is being referred to. Qualifying the name with that of the containing class can make the code clearer.
            this.response = copyOf(messages).stream()
                            ^
    (see https://errorprone.info/bugpattern/BadImport)
  Did you mean 'this.response = ImmutableSet.copyOf(messages).stream()'?
/Users/foobar/development/Spine/web/firebase-web/src/test/java/io/spine/web/firebase/HasChildren.java:56: warning: [BadImport] Importing nested classes/static methods/static fields with commonly-used names can make code harder to read, because it may not be clear from the context exactly which type is being referred to. Qualifying the name with that of the containing class can make the code clearer.
        this.expected = copyOf(expected);
                        ^
    (see https://errorprone.info/bugpattern/BadImport)
  Did you mean 'this.expected = ImmutableMap.copyOf(expected);'?
/Users/foobar/development/Spine/web/firebase-web/src/test/java/io/spine/web/firebase/FirebaseClientFactoryTest.java:36: warning: [BadImport] Importing nested classes/static methods/static fields with commonly-used names can make code harder to read, because it may not be clear from the context exactly which type is being referred to. Qualifying the name with that of the containing class can make the code clearer.
    private static final DatabaseUrl SOME_URL = from("https://someUrl.com");
                                                ^
    (see https://errorprone.info/bugpattern/BadImport)
  Did you mean 'private static final DatabaseUrl SOME_URL = DatabaseUrl.from("https:'?
3 warnings