objecter: configurable num of completion locks per session

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
objecter: fix command ops handling

Had some locking issues. Now command ops are basically doing the same as
the regular and lingering ops. Separated the recalc function into two
functions, one that calculates the target and the second one that
assigns the session.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>