Feature: use nonce to add attachment timestamp.

[alon-e]

NONCE_TRINARY_SIZE = 243;

will be changed to

TAG_TRINARY_SIZE = 81
ATTACHMENT_TIMESTAMP_TRINARY_SIZE = 27
ATTACHMENT_TIMESTAMP_LOWER_BOUND_TRINARY_SIZE = 27
ATTACHMENT_TIMESTAMP_UPPER_BOUND_TRINARY_SIZE = 27
NONCE_TRINARY_SIZE = 81

to allow specifying a timestamp for each attached transaction (so re-attachments will have a different attachment timestamp).
also the tag has been moved out to the attachment section, to allow a different tag for each re-attachment.

in this PR, the tag is the same as obsolete tag.

[alon-e]

yes, currently the value of (attachment) TAG will be the same as (bundle) TAG.
as this is what is impl. in IRI's attachToTangle API call.

however, this opens the option to assign a different TAG to a single instance of the attached bundle in the future.