MM-11262: database config store (#10281)

* vendor github.com/jmoiron/sqlx

* MM-11262: introduce a database store

* revert unnecessary fmt.Errorf

* simplify unit test helper methods

* remote TODO re: retry

* relocate initializeConfigurationsTable for clarity

* factor out a commonStore

* acquire database config lock on close for safety

* add missing header

* fix lock comment

vendor/github.com/jmoiron/sqlx/reflectx/README.md

@@ -0,0 +1,17 @@
+# reflectx
+
+The sqlx package has special reflect needs.  In particular, it needs to:
+
+* be able to map a name to a field
+* understand embedded structs
+* understand mapping names to fields by a particular tag
+* user specified name -> field mapping functions
+
+These behaviors mimic the behaviors by the standard library marshallers and also the
+behavior of standard Go accessors.
+
+The first two are amply taken care of by `Reflect.Value.FieldByName`, and the third is
+addressed by `Reflect.Value.FieldByNameFunc`, but these don't quite understand struct
+tags in the ways that are vital to most marshallers, and they are slow.
+
+This reflectx package extends reflect to achieve these goals.