[dss] Reduce contention on OI creation/mutation endpoint

[barroco]

This PR adds a lock at the beginning of the transactions creating or mutating operational intents.
Contention is mainly due to the ability of the transaction to increment the subscription's notification index.
The lock will ensure transactions with contention on subscriptions on the same cells to wait before starting preventing multiple retries.

Compared to the result reported in the issue #1002, we can identify the following overall test durations (single run) and number of transaction retries:

test step	v0.11.0-rc1	#1004
test_create_ops_concurrent	4s	1.4 s
test_mutate_ops_concurrent	5.2s	1.5 s
overall retries	150	17
overall CI run	~ 24s	~ 19s

The following diagram shows the result of scd/test_operation_simple_heavy_traffic_concurrent.py. Test was run on a GCP deployment (3 core-services, 3 cockroach db nodes):

This improvement should address #742.

Note that due to the sensitivity of the change, I tried to keep it as focused as possible. Some opportunities for refactoring will be addressed separately.

[BenjaminPelletier]

It's great we can sufficiently alleviate the problem with a solution this simple -- thanks for doing this!

After this P0 is resolved, we can (at a more normal priority) take a look at those follow-up items like refactoring and additional improvements. Thanks again!