Implement proper handling of agenda markers when using sticky agendas

Implement proper handling of agenda markers when using sticky agendas

* lisp/org-agenda.el (org-toggle-sticky-agenda): Kill all agenda buffers
when toggling sticky-agendas.
(org-agenda-get-restriction-and-command): Add `C-c a C-k' as a key to
explicitly kill all agenda buffers.
(org-agenda-run-series): Remove any old agenda markers in the buffer
that is going to take the new block agenda.
(org-prepare-agenda): Reset markers before erasing the buffer anc
running `org-agenda-mode', because after that hte local variable
`org-agenda-markers' will have gone away.
(org-agenda-Quit):
(org-finalize-agenda): Install the marker resetter into
the `kill-buffer-hook'.
(org-agenda-save-markers-for-cut-and-paste): Look for markers in all
agenda buffers.
(org-agenda-kill-all-agenda-buffers): New function.