Fix overflow issues in seqNr and ackNr

[acolytec3]

This addresses #691.

The Problem

uTP sequence numbers are 16 bit unsigned integers so have a range of 0-65535. When a sequence number reaches the maximum value, we would expect the next sequence number to be 0. Ultralight was not checking for overflow and blithely continuing on in the sequence since we are using native Javascript numbers in our code rather than true 16-bit intengers.

Solution

• Revise contentWriter sodataChunks are stored in an array of tuples of the form [seqNr, bytes] rather than an object keyed by the seqNr
• Check for overflow in all sequence and ack number checks and updates
• Convert some Object.keys code to simple Array accesses (which should provide some small performance boost)
• Removes writeSocket.dataNrs as unnecessary

[ScottyPoi]

LGTM