**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.


MergeableBlocksPtr getMergeableBlocks() { return mergeable_blocks; }
    
    MergeableBlocksPtr collectMergeableBlocks(const Context & context);
    
    BlockInputStreamPtr completeQuery(Pipes pipes);
    void setMergeableBlocks(MergeableBlocksPtr blocks) { mergeable_blocks = blocks; }
    std::shared_ptr<bool> getActivePtr() { return active_ptr; }
    
    bool getNewBlocks();
    Block getHeader() const;
    
    static Pipes blocksToPipes(BlocksPtrs blocks, Block & sample_block);
    static void writeIntoLiveView(
        StorageLiveView & live_view,
        const Block & block,
        const Context & context);
private:
    
    StorageID select_table_id = StorageID::createEmpty();     
    ASTPtr inner_query; 
    ASTPtr inner_subquery; 
    ASTPtr inner_blocks_query; 
    Context & global_context;
    std::unique_ptr<Context> live_view_context;
    bool is_temporary = false;
    
    mutable std::mutex sample_block_lock;