[ecash-herald] Limit number of displayed xec send txs

Summary:
Some large blocks create very long msgs, which take multiple telegram msgs. A general principle is that we want each message to be as short and as interesting as possible.

Do not show too many eCash send msgs in the block. This diff simply displays the 12 biggest xec send txs. Better filtering can be introduced going fwd (for example, mb we want only whale wallet sends, or only known addresses, etc...some formula to get the most interesting txs).

Test Plan: npm run sendMsgByBlock 799298 output is pictured.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D14204