v3 (erc-display-line redux). Properly offset renarrowed region after
inserting initial date stamp in `erc-insert-timestamp-left-and-right'.
Don't displace third-party markers when inserting left-sided stamps in
`erc-stamp--display-margin-mode'.

The first bug was introduced by

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

and causes right-sided stamps to appear inside the prompt, among other
unpleasant things (see third patch). Thanks to Corwin for spotting this.
The other bug has been around a bit longer, likely since

  63d8b2a59a4 * Make erc-fill-wrap work with left-sided stamps

It has the potential to break packages that place markers in
modification hooks (see last patch).