Adding new Mutation constructor with ByteSequence parameter to remove 1 TODO

[Amemeda]

Removing TODO add ByteSequence methods to mutations from line 63 of ConditionalMutation.java

Updated Mutation.java

• Added new constructor with ByteSequence row parameter
• Added new methods init and copyIfNeeded based on implementation from Key.java
• Added new get method to get row as a ByteSequence

Updated MutationTest.java

• Added new test testByteSequenceConstructor

[Amemeda]

Removes 1 TODO from #2699

[dlmarion]

I'm assuming the new constructor would be called here too. I'm curious how many places in the code would take advantage of the new constructor for the amount of code that it adds to Mutation. If there are not many places, then I think it might be better to just remove the TODO.