plume-model: refactor Searcher to have its own DbPool

this way, we don't need to pass along a conn into the function.
This should make splitting PlumeRocket up into its components a little
easier. This PR is another attempt at fixing #799.

         let lang = schema.get_field("lang").unwrap();
         let license = schema.get_field("license").unwrap();
 
+        let conn = self.dbpool.get().unwrap();

         let searcher = self.reader.searcher();
         let res = searcher.search(&query.into_query(), &collector).unwrap();
 
+        let conn = self.dbpool.get().unwrap();

     Io(std::io::Error),
     MissingApProperty,
     NotFound,
+    DbPool,

 
+        let conn = match self.dbpool.get() {
+            Ok(c) => c,
+            Err(_) => return Err(Error::DbPool),

+    ///
+    /// The panic messages are normally explicit enough for a human to
+    /// understand how to fix the issue when they see it.
+    pub fn new(db_pool: DbPool) -> Arc<Self> {