Replace use of `Duration` for timestamps with an actual timestamp, e.g. `chrono::DateTime`

[0x009922]

In some places in iroha_data_model, structures use Duration for data which doesn't mean a span of time, but actually means a timestamp. Examples:

• start: https://github.com/hyperledger/iroha/blob/4362c9427fa8b4090676ac46e8171ef5f1995ff6/data_model/src/events/time.rs#L100-L105
• since: https://github.com/hyperledger/iroha/blob/4362c9427fa8b4090676ac46e8171ef5f1995ff6/data_model/src/events/time.rs#L126-L131

These fields are timestamps, not time spans.

I propose to use a structure that is designed specifically for timestamps, e.g. chrono::DateTime.

[hollermay]

Can I work on this issue? I can help around this

[nxsaken]

Hi @hollermay, feel free to open a PR.

[mversic]

start should become start_ms and since should become since_ms. However, getters should return something like chrono::DateTime

@hollermay should I assign you?

[hollermay]

start should become start_ms and since should become since_ms. However, getters should return something like chrono::DateTime

@hollermay should I assign you?

Sure I can give a shot

[mversic]

also do the same for SignedTransactionV1::creation_time_ms and SignedBlockV1::timestamp_ms

[hollermay]

@mversic please check the pull request