Improve cleanup_index_files

[iilyak]

Overview

The previous implementation was based on a search using

{view_index_dir}/.shards/*/{db_name}.[0-9]*_design/mrview/*

This wildcard includes all shards for all indexes of all databases.
This caused performance issues in the case when there are thousands of databases on the node.
This PR changes the search to look at index_directory of a database.

Testing recommendations

make && make eunit apps=couch tests=mrview_cleanup_index_files_test_

Related Issues or Pull Requests

N/A

Checklist

• Code is written and works correctly;
• Changes are covered by tests;
• Documentation reflects the changes;

[jaydoane]

test passes locally:

$ make eunit apps=couch tests=mrview_cleanup_index_files_test_
==> couch (eunit)
    Running test function(s):
      couchdb_mrview_tests:mrview_cleanup_index_files_test_/0
======================== EUnit ========================
Check index files cleanup
  clustered
    couchdb_mrview_tests:146: should_cleanup_index_files...[0.210 s] ok
    [done in 0.215 s]
=======================================================
  Test passed.

[jaydoane]

LGTM, after fixing indentation

[jaydoane]

The indentation here is inconsistent, and doesn't match the other tests in the file (which use 4 spaces per indent).