v2. Fix date-stamp regression in erc-track. Optionally reinstate old
"prepended" date-stamp behavior gated by new compat var.

Earlier changes for this feature introduced a regression involving date
stamps and the option `erc-track-exclude-types'. Basically, date stamps
aren't supposed to affect the mode line, at least so long as their
inciting message's command appears in `erc-track-exclude-types'.
However, this changed after

  c68dc7786fc * Manage some text props for ERC insertion-hook members

To reproduce from -Q:

  1. Connect and ensure "JOIN" appears in `erc-track-exclude-types'
  2. Join #chan
  3. From the server buffer, do

     (with-current-buffer "#chan"
       (setq erc-timestamp-last-inserted-left nil))

  3. Connect and join #chan from another client
  4. Notice a [#c] in the mode line of the original client

Thanks to Corwin for pointing this out. The way I'm proposing we tackle
this is to decouple date stamps from `erc-track-exclude-types'
completely. That is, have erc-track completely ignore them, so they
never affect the mode line.

In addition to this fix, I've also added a path for accessing the old
behavior in which date stamps aren't standalone messages.