Handle context.DeadlineExceeded when retrying a transaction

[peterjan]

Caught these errors on renterd shutdown, fixed by adding a context.DeadlineExceeded case in abortRetry.

2024-04-23T10:49:08Z INFO  bus  successfully saved 392 accounts
2024-04-23T10:49:42Z ERROR SQL  trace {"error": "context deadline exceeded", "elapsed": "34.701151966s", "rows": 0, "sql": "\nDELETE\nFROM slabs\nWHERE NOT EXISTS (SELECT 1 FROM slices WHERE slices.db_slab_id = slabs.id)\nAND slabs.db_buffered_slab_id IS NULL\n"}
2024-04-23T10:49:42Z WARN  sql  transaction attempt 1/7 failed, retry in 200ms,  err: context deadline exceeded
2024-04-23T10:49:42Z WARN  sql  transaction attempt 2/7 failed, retry in 500ms,  err: context deadline exceeded
2024-04-23T10:49:43Z WARN  sql  transaction attempt 3/7 failed, retry in 1s,  err: context deadline exceeded
2024-04-23T10:49:44Z WARN  sql  transaction attempt 4/7 failed, retry in 3s,  err: context deadline exceeded
2024-04-23T10:49:47Z WARN  sql  transaction attempt 5/7 failed, retry in 10s,  err: context deadline exceeded
2024-04-23T10:49:57Z WARN  sql  transaction attempt 6/7 failed, retry in 10s,  err: context deadline exceeded