Undo the lenient MIME BASE64 decoder support change (JDK-8025003) and remove de/encode(buf, buf) methods

The change set for "lenient mime decoder" support change (JDK-8025003) does not meet the requirement needed by the submitter and there is different understanding and opinion of what the "lenient" should be for the Base64 decoder. Given we are in late stage of the release. The decision is to back out the change and leave the functionality for future releases.

Also during the discussion, it became clear that the decode/encode(ByteBuffer, ByteBuffer) method pair are insufficient to support the requested de/encoding continuation functionality ("underflow" handling, in charset coder's term). Given the the most usage of these pair is for the channel read/writing, it appears it might be more useful/desired to provide a pair of wrap(readable/writableByteChannel) methods stead. While the wrap(channel) methods are under development, it does not appear it will make jdk8. For jdk8 we have decided to remove the decode/encode(Buffer, Buffer).