added test for PIndexOpen() failure case
TODO comment on checking PIndex metadata during open
comment on how bleve handles Close() and concurrency

The top-level API of bleve conveniently provides out-of-the-box
concurrency protection, which was an answer heard directly from
bleve's "Blevevolent Dictator"...

https://groups.google.com/d/msg/bleve/NsK4acSEp3M/8rkfdWXsnpkJ