The Test Case Importer should support concurrent imports

In Test Case Importer, when doing concurrency imports (two users at the same time) only one import will be succeeded.

Steps to produce the issue:
1. prepare a CSV file that contains more than 2k test cases similar to the one attached below
2. User A imports the test cases via Xray's Test Case Importer
3. While step 2 is still in progress, then User B uploads his/her CSV file.

Expected result:
User B will see the importing progress starting from 9% and the test cases will be imported.

Actual result:
User B can only see the importing progress of User A's importing and his/her test cases are not imported.