Timestamp encoding to bytes is weird

[vedhavyas]

ATM we encode bytes as follows

• Seconds(8 bytes) + nanoseconds(4 bytes)

We can either remove the nanosecond completely and just encode the epoch until seconds.
If we want to nano seconds to be included, then use time.UnixNano() to fetch the uin64 and encode that to bytes.

this will break the current AssetNFTs that rely on the maturity date in the above-mentioned format.