[imp] large session entries in horde_sessionhandler

[liam hoekenga]

> Right now, this is mandatory for IMP to work properly (and has been
> throughout all IMP 4.x releases - this is not something that has been
> recently added).  Search the mailing lists a year or two back for the
> very long explanation I gave about why this is necessary (a search for
> "embedded MIME parts" will probably return the results you need).
> Quick explanation: caching rendered parts is the only way we currently
> can pass embedded MIME part data to another page without having to
> write hundreds of lines of rendering code in each IMP page.

Can we safely reduce the cache size from 20 to a smaller number (10? 5?)

Liam