[1396] Rename method to getLength()

At first I thought getLength() may already be in use by RR type-specific
classes, but it's not in use. getLength() is more consistent with naming
of other classes' wire data length methods.
[1396] Add RRset::getLength()
[1396] Add Python binding for RRset::getLength()
[1396] Add TreenodeRRset::getLength() implementation
[1396] Fix width of TTL field
[1396] Use RRset::getLength() in xfrout