Review/fix/change file reassembly functionality.

- Re-arrange how some fa_file fields (e.g. source, connection info, mime
  type) get updated/set for consistency.

- Add more robust mechanisms for flushing the reassembly buffer.
  The goal being to report all gaps and deliveries to file analyzers
  regardless of the state of the reassembly buffer at the time it has to
  be flushed.

src/file_analysis/Manager.h

@@ -319,6 +319,7 @@ protected:
 	 *        this file isn't related to a connection).
 	 * @param update_conn whether we need to update connection-related field
 	 *        in the \c fa_file record value associated with the file.
+	 * @param an optional value of the source field to fill in.
 	 * @return the File object mapped to \a file_id or a null pointer if
 	 *         analysis is being ignored for the associated file.  An File
 	 *         object may be created if a mapping doesn't exist, and if it did
@@ -327,7 +328,8 @@ protected:
 	 */
 	File* GetFile(const string& file_id, Connection* conn = 0,
 	              analyzer::Tag tag = analyzer::Tag::Error,
-	              bool is_orig = false, bool update_conn = true);
+	              bool is_orig = false, bool update_conn = true,
+	              const char* source_name = 0);
 
 	/**
 	 * Try to retrieve a file that's being analyzed, using its identifier/hash.