[explorer] Use timeFirstSeen instead of block timestamp for tx table time

Summary:
Right now the explorer uses timeFirstSeen for txs that are unconfirmed, i.e. do not have a blockheight. For txs with a blockheight, it uses the block timestamp.

This is perhaps more "right," but it is not as useful as timeFirstSeen.

For example, when I check the token server wallet, it is not useful at all if I see 20 txs with the same timestamp. Maybe they are all in the same block which took an hour to find.

However if I see 20 txs that are all coming a few seconds apart, this is a good sign there is some kind of a script running.

timeFirstSeen imo provides more useful information.

Test Plan:
preview, check out https://explorer.e.cash/address/ecash:qzppgpav9xfls6zzyuqy7syxpqhnlqqa5u68m4qw6l to see txs with the same blockheight getting slightly different timestamps

can check out http://localhost:3035/address/ecash:qzuec4eff39mzjwh9gcqxee4qjfy5ns0wqlth2k995?page=2 (random address I found by looking up block 500,000 to get txs this node would not have timeFirstSeen for) to see txs with same blockheight get the same timestamp when timeFirstSeen is 0

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D16751