init: initialize spack.store lazily

- spack.store was previously initialized at the spack.store module level,
  but this means the store has to be initialized on every spack call.

- this moves the state in spack.store to a singleton so that the store is
  only initialized when needed.

var/spack/repos/builtin/packages/openspeedshop/package.py

@@ -197,7 +197,8 @@ def set_CrayLoginNode_cmakeOptions(self, spec, cmakeOptions):
         # spec['cbtf'].prefix is the login node value for this build, as
         # we only get here when building the login node components and
         # that is all that is known to spack.
-        be_ck = spack.store.db.query_one('cbtf-krell arch=cray-CNL-haswell')
+        store = spack.store.store()
+        be_ck = store.db.query_one('cbtf-krell arch=cray-CNL-haswell')
 
         # Equivalent to install-tool cmake arg:
         # '-DCBTF_KRELL_CN_RUNTIME_DIR=%s'