storage: add state to bind
storage: generalize lockers

We will need different lockers here.
vcl_writer: use the new locking scheme
storage: make load_units more generic

Now we can provide the query filters, instead of just the state. The
same thing is coming for binds.
storage: unify retrieve_units and load_units
storage: generalize retrieve_binds
storage: split MongoDBStorage and MultiLocker
storage: rename init_locker to just init

Since now we have a MultiLocker type, it can be just init.
storage: support limit in retrieve_binds
storage: add method to update bind
vcl_writer: fix behavior

Now the writer writes the VCL whenever a new unit is added or a new bind
is made.
feaas: bump to 0.5.7