Making AsciiSequenceView not final

[RichardWarburton]

In a javadoc improvement commit (d6322f7) AsciiSequenceView was made final. We make use of that class in Artio and it looks like some users have subclassed it, making Agrona 1.10 an incompatible upgrade for them. Would there be any objection to allowing subclassing on that class again?

I've poked the user on their motivation here, but the thing is that because AsciiSequenceView instances can relate to fields with FIX messages I think that if you want to have a field with some extra validation of logic on it and not pay an additional indirection price from composing your field class with an AsciiSequenceView then sub-classing seems reasonable.