Better exclusive lock for merge trains

Problem to solve

Currently, train processor locks the thread until it's finished refreshing all the following merge requests in a train.

This is safe, however, could cause a race condition in the following case:

RefreshMergeRequestsService: PID-1
Case: MR3 pipeline finished

               v
| MR1 | MR2 | MR3 | MR4 | MR5 |

RefreshMergeRequestsService: PID-2
Case: MR1 pipeline finished

   v <= should be processed
| MR1 | MR2 | MR3 | MR4 | MR5 |

We should process MR1 in PID-2 in the above case, otherwise, MR1 won't be merged.