Propose a scheme for "allocating" txs into Leios Input Blocks

[nfrisby]

With the abrupt end of the Well-Typed contract, some expert opinions on this open Leios design question were lost (see #1509).

This Issue is my driving motivator to establish a useful opinion on this design question.

Specifically: how should Cardano "usher" transactions submitted by users to Input Blocks? The scope of this question also includes tuning IB/EB/RB rates, expiry ages, etc.