util/mysql: terminate DEBUG log messages with new line
test/indexing: Add test_mysql_search_existing_fmid_invalid_input

Also restructure a bit the indexing testsuite so now we have:
- test case for MySQL backend initialization
- test case for MySQL backend internal implementation
- test case to test MySQL backend throught indexing interface
backend/indexing: Add more sanity checks and comments for mysql_search_existing_fmid

Also rename IsSoftDeleted to is_soft_deleted more in the coding
style in OpenChange