Fix concurrent OPTIMIZE and DROP for ReplicatedMergeTree

Found with fuzzer [1] for 00992_system_parts_race_condition_zookeeper:

    2021.03.13 11:12:30.385188 [ 42042 ] {2d3a8e17-26be-47c1-974f-bd2c9fc7c3af} <Debug> executeQuery: (from [::1]:58192, using production parser) (comment: '/usr/share/clickhouse-test/queries/1_stateful/00153_aggregate_arena_race.sql') CREATE TABLE alter_tabl
    e (a UInt8, b Int16, c Float32, d String, e Array(UInt8), f Nullable(UUID), g Tuple(UInt8, UInt16)) ENGINE = ReplicatedMergeTree('/clickhouse/tables/test_3.alter_table', 'r1') ORDER BY a PARTITION BY b % 10 SETTINGS old_parts_lifetime = 1, cleanup_delay_p
    eriod = 1, cleanup_delay_period_random_add = 0;
    ...
    2021.03.13 11:12:30.678387 [ 42042 ] {528cafc5-a02b-4df8-a531-a9a98e37b478} <Debug> executeQuery: (from [::1]:58192, using production parser) (comment: '/usr/share/clickhouse-test/queries/1_stateful/00153_aggregate_arena_race.sql') CREATE TABLE alter_table2 (a UInt8, b Int16, c Float32, d String, e Array(UInt8), f Nullable(UUID), g Tuple(UInt8, UInt16)) ENGINE = ReplicatedMergeTree('/clickhouse/tables/test_3.alter_table', 'r2') ORDER BY a PARTITION BY b % 10 SETTINGS old_parts_lifetime = 1, cleanup_delay_period = 1, cleanup_delay_period_random_add = 0;
    ...
    2021.03.13 11:12:40.671994 [ 4193 ] {d96ee93c-69b0-4e89-b411-16c382ae27a8} <Debug> executeQuery: (from [::1]:59714, using production parser) (comment: '/usr/share/clickhouse-test/queries/1_stateful/00153_aggregate_arena_race.sql') OPTIMIZE TABLE alter_table FINAL
    ...
    2021.03.13 11:12:40.990174 [ 2298 ] {a80f9306-3a73-4778-a921-db53249247e3} <Debug> executeQuery: (from [::1]:59768, using production parser) (comment: '/usr/share/clickhouse-test/queries/1_stateful/00153_aggregate_arena_race.sql') DROP TABLE alter_table;
    ...
    2021.03.13 11:12:41.333054 [ 2298 ] {a80f9306-3a73-4778-a921-db53249247e3} <Debug> test_3.alter_table (d4fedaca-e0f6-4c22-9a4f-9f4d11b6b705): Removing part from filesystem 7_0_0_0
    ...
    2021.03.13 11:12:41.335380 [ 2298 ] {a80f9306-3a73-4778-a921-db53249247e3} <Debug> DatabaseCatalog: Waiting for table d4fedaca-e0f6-4c22-9a4f-9f4d11b6b705 to be finally dropped
    ...
    2021.03.13 11:12:41.781032 [ 4193 ] {d96ee93c-69b0-4e89-b411-16c382ae27a8} <Debug> test_3.alter_table (d4fedaca-e0f6-4c22-9a4f-9f4d11b6b705): Waiting for queue-0000000085 to disappear from r2 queue
    ...
    2021.03.13 11:12:41.900039 [ 371 ] {} <Trace> test_3.alter_table2 (ReplicatedMergeTreeQueue): Not executing log entry queue-0000000085 of type MERGE_PARTS for part 7_0_0_1 because part 7_0_0_0 is not ready yet (log entry for that part is being processed).
    2021.03.13 11:12:41.900213 [ 365 ] {} <Trace> test_3.alter_table2 (ReplicatedMergeTreeQueue): Cannot execute alter metadata queue-0000000056 with version 22 because another alter 21 must be executed before
    2021.03.13 11:12:41.900231 [ 13762 ] {} <Trace> test_3.alter_table2 (ae877c49-0d30-416d-9afe-27fd457d8fc4): Executing log entry to merge parts -7_0_0_0 to -7_0_0_1
    2021.03.13 11:12:41.900330 [ 13762 ] {} <Debug> test_3.alter_table2 (ae877c49-0d30-416d-9afe-27fd457d8fc4): Don't have all parts for merge -7_0_0_1; will try to fetch it instead
    ...

  [1]: https://clickhouse-test-reports.s3.yandex.net/21691/eb3710c164b991b8d4f86b1435a65f9eceb8f1f5/stress_test_(address).html#fail1

src/Storages/StorageReplicatedMergeTree.cpp

@@ -4151,6 +4151,10 @@ bool StorageReplicatedMergeTree::optimize(
     const Names & deduplicate_by_columns,
     const Context & query_context)
 {
+    /// NOTE: exclusive lock cannot be used here, since this may lead to deadlock (see comments below),
+    /// but it should be safe to use non-exclusive to avoid dropping parts that may be required for processing queue.
+    auto table_lock = lockForShare(query_context.getCurrentQueryId(), query_context.getSettingsRef().lock_acquire_timeout);
+
     assertNotReadonly();
 
     if (!is_leader)