fix(issue summary): Add locking mechanism

[roaga]

If a summary is in progress for an issue while another request comes in, the other request should wait for a bit first. This ensures only one call to Seer per issue at a time.

[codecov]

❌ 1 Tests Failed:

Tests completed	Failed	Passed	Skipped
25296	1	25295	306

View the top 1 failed test(s) by shortest run time

tests.sentry.seer.test_issue_summary.IssueSummaryTest::test_get_issue_summary_concurrent_wait_for_lock

Stack Traces | 3.68s run time

#x1B[1m#x1B[.../sentry/seer/test_issue_summary.py#x1B[0m:453: in test_get_issue_summary_concurrent_wait_for_lock
    assert 1 in results
#x1B[1m#x1B[31mE   assert 1 in {}#x1B[0m

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