mon: make service trim_to stateless

mon: make service trim_to stateless

Call get_trim_to() when we need to know how much to trim (if any), and
calculate it then.  No need to keep this in a hidden trim_version
variable and remember to update it.  This drops several helpers and
accessors and makes get_trim_to() a single method that services need to
override.

Signed-off-by: Sage Weil <sage@inktank.com>