Fixes #608: Random crash when changing folder

[marbetschar]

The crash described in #608 is caused by some asynchronous voodoo, where folders[service_uid] is already freed at the time when get_search_result_uids () tries to access it.

This PR mitigates the issue by adding the following safety measures:

1. Set folders[service_uid] = folder before binding to folder.changed signal - just to be sure (swapped lines 146 and 147)
2. get_search_result_uids function locks the folders HashMap during its executing to prevent changes
3. get_search_result_uids function checks for existence of folders[service_uid] - if it does not exists, it returns null
4. Callers of the get_search_result_uids function handle the null case gracefully

[danirabbit]

I was able to reproduce a crash in master without this branch and cannot reproduce it with this branch. Nice work :)