**Describe the bug**
Using `ALTER LIVE VIEW view REFRESH` results in an exception.

**How to reproduce**
* Connected to ClickHouse server version 20.8.1 revision 54438
* `CREATE TABLE` statements for all tables involved
```
clicktest :) SHOW CREATE table0

SHOW CREATE TABLE table0

┌─statement─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┐
│ CREATE TABLE default.table0
(
    `d` Date,
    `a` String,
    `b` UInt8,
    `x` String,
    `y` Int8
)
ENGINE = MergeTree()
ORDER BY d
SETTINGS index_granularity = 8192 │
└───────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘

1 rows in set. Elapsed: 0.006 sec.
```
* Queries to run that lead to unexpected result
```
ClickHouse client version 20.8.1.4447 (official build).
Connecting to localhost:9000 as user default.
Connected to ClickHouse server version 20.8.1 revision 54438.

clicktest :) SET allow_experimental_live_view=1

SET allow_experimental_live_view = 1

Ok.

0 rows in set. Elapsed: 0.003 sec.

clicktest :) CREATE LIVE VIEW live1 AS SELECT * FROM table0

CREATE LIVE VIEW live1 AS
SELECT *
FROM table0

Ok.

0 rows in set. Elapsed: 0.008 sec.

clicktest :) ALTER LIVE VIEW live1 REFRESH

ALTER LIVE VIEW live1
    REFRESH

Received exception from server (version 20.8.1):
Code: 49. DB::Exception: Received from localhost:9000. DB::Exception: RWLockImpl::getLock(): RWLock is already locked in exclusive mode.

0 rows in set. Elapsed: 0.005 sec.
```
**Expected behavior**
A clear and concise description of what you expected to happen.


{
        (*blocks_ptr).reset();
        if (*blocks_metadata_ptr)
            (*blocks_metadata_ptr)->hash.clear();
        mergeable_blocks.reset();
    }
    void checkTableCanBeDropped() const override;
    void drop() override;
    void startup() override;
    void shutdown() override;
    void refresh(const Context & context);
    Pipe read(
        const Names & column_names,
        const StorageMetadataPtr & ,
        const SelectQueryInfo & query_info,
        const Context & context,
        QueryProcessingStage::Enum processed_stage,
        size_t max_block_size,
        unsigned num_streams) override;
    BlockInputStreams watch(
        const Names & column_names,
        const SelectQueryInfo & query_info,
        const Context & context,
        QueryProcessingStage::Enum & processed_stage,
        size_t max_block_size,
        unsigned num_streams) override;
    std::shared_ptr<BlocksPtr> getBlocksPtr() { return blocks_ptr; }