don't let the environment override the INCDIR and LIBDIR for dependencies
allow to assign a label to the shardcache nodes and wrap the counters in a context

having labels (and sharding based on those) makes possible to change ip address or dns name
to nodes without affecting the key distribution (since it's now based on the label,
which shouldn't be changed instead)

wraping the counters in a context (opaque) structure instead makes possible to
instantiate multiple shardcache instance in the same process.
aligned the perl bindings to the last changes in libshardcache