Use connection pool for redis dictionary

[CurtizJ]

Changelog category (leave one):

• Bug Fix (user-visible misbehaviour in official stable or prestable release)

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):
Fix usage of external dictionaries with Redis source and large number of keys.

Fixes #27784.

[CurtizJ]

BTW, performance of redis dictionaries is still questionable. Maybe we need to use another client library for redis. But ok for now.

[kitaisreal]

Consider to use ConnectionPtr as return type, without this auto.

[CurtizJ]

This construction helps to avoid extra qualification for ConnectionPtr. Otherwise we will have to write

RedisDictionarySource::ConnectionPtr RedisDictionarySource::getConnection() const

But I'm ok to replace this.

[kitaisreal]

Not sure maybe it is better to backport only crash fix, without adding pool.

[CurtizJ]

BTW, performance of redis dictionaries is still questionable.

More precisely, for direct dictionary it's about 300-400K rows for SELECT * FROM dict and about 60-80K rows for dictGet with complex key dictionary.

[kitaisreal]

@CurtizJ lets merge it that way, later we can investigate how performance can be improved.