fix(java): stop Show Usages waiting seconds on the configuration index after a change (#2011)

Usages counts clicked after a Java change waited for the configuration index to resolve every dependency's Spring metadata again (15–20 s in a Spring Boot project). The Java extension (1.8.0) now keeps each dependency JAR's metadata across rebuilds until the JAR or the classpath changes, and Show Usages shows the Java usages first, adding the configuration ones when the index takes longer.

Closes #1898

java/java-usages/CONFIGURATION.md

@@ -78,7 +78,13 @@ changes; a fingerprint of working copies with unsaved changes covers edits befor
 they are saved. Opening or closing an editor, reconciling, building, and non-Java
 resource changes (configuration files are parsed by the client; metadata files are
 checked by timestamp) keep it. This is **snapshot invalidation**, not a fully
-incremental Java semantic index.
+incremental Java semantic index. A rebuild reuses each dependency JAR's metadata
+associations (up to 100,000 in all) while the JAR's timestamp and size and the project's
+classpath are unchanged: resolving a Spring Boot JAR's thousands of properties to library
+members takes seconds, and a source change cannot affect them.
+
+Find Usages shows the Java usages as soon as they are found, then adds the configuration
+ones when a rebuild of the snapshot takes longer.
 
 In YAML/properties, ⌘/Ctrl-hover underlines the whole key, hyphens and dots
 included, and doesn't open the documentation popup. Hover and Go to Definition share