insight: set time with MempoolTime from mempool tx store

[chappjc]

For unconfirmed transactions, this sets a non-zero time value for the following Insight API endpoints:

• /insight/api/addrs/{address}/txs
• /insight/api/txs?address={address}

Note that this sets the "time" JSON value for unconfirmed transactions only. After confirmation, both "time" and "blocktime" will be set to the value from the getrawtransaction RPC (the mempool time is no longer known).

[buck54321]

There's a chance, however small, that someone might be using the absence of a time field as an indicator of lack of confirmation. Is this a breaking change, and if so does it require any special versioning?

[chappjc]

Hmm, that change of behavior could be breaking, but I'm inclined to ignore that possibility since a client doing that is essentially abusing a bug. We're also not versioning the Insight API in any way (either with a /v1 url prefix or other mechanism), so there are essentially no promises, which BTW is also what the official Insight API seems to do. At least one of the recent changes to our Insight API implementation were just to keep up with some random changes made to bitpay's Insight implementation.

This reminds me of one of the oldest issues, "Version the API", which refers to our own API. We're essentially treating the API as having v0 status, but that should change when we feel no more changes are needed.