Revert "PaxosService: use get and put for version_t"

This reverts commit e725c3e210b244e090d70c77d937c94f4f63a2be.

These inadvertantely got rid of the prefix portion of the key, which
lead to overwriting the wrong keys.

Fixes: #4872
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Samuel Just <sam.just@inktank.com>