block in catalog if someone tries to add something early

fixes the intermittent error we've been getting, such as:

https://buildhive.cloudbees.com/job/brooklyncentral/job/brooklyn/2413/io.brooklyn$brooklyn-core/testReport/brooklyn.entity.rebind/RebindCatalogEntityTest/testRestoresAppFromCatalogClassloader/

does so by blocking if the catalog is not yet fully loaded when someone tries to add something manually to it.  there are better fixes as noted in the code (manual additions could be done in bg) but this is a quicker pragmatic workaround.
No consensus, but I try and follow it, as the spiritual successor to the Sun Microsystems guidelines. As part of the incubation process we can decide on a suitable standard.