[IMAPx] Speed-up folder cache removal on folder sync

This used to save store's summary after each single folder removal,
which includes also an fsync() call. Just remove unknown folders and
save changes once significantly speeds-up the initial folder hierarchy
sync with many folder removals.
[IMAPx] Enhance and make work namespace override options

The namespace override didn't work at all, thus made it work first,
then added two new options to ignore other users namespace and
shared folders namespace. All these options modify server-supplied
list of namespaces.