… in one DSN (nicolas-grekas)

This PR was merged into the 4.2-dev branch.

Discussion
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[Cache] support configuring multiple Memcached servers in one DSN

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #27855
| License       | MIT
| Doc PR        | symfony/symfony-docs#10402

Useful to reconfigure dynamically an array of memcached servers (eg removing a dead one or adding a new one).
DSN format is e.g. `memcached://localhost?host[foo.bar]=3`.
To ease generating the DSN programmatically, it works also with `memcached:?host[localhost]&host[localhost:12345]&host[/some/memcached.sock:]=3`.

The key of the "host" parameter is a "host:port" pair, the value is the weight of the "host:port" pair.
Sockets need to be specified with the trailing `:` (as shown in the last example).

Commits
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8e0605a [Cache] support configuring multiple Memcached servers in one DSN

… via DSN (nicolas-grekas)

This PR was merged into the 4.2-dev branch.

Discussion
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[Cache] added support for connecting to Redis clusters via DSN

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Replaces #28300 and #28175

This PR allows configuring a cluster of Redis servers using all available options of either the phpredis extension or the Predis package:
- the `redis_cluster=0/1` boolean option configures whether the client should use the Redis cluster protocol;
- several hosts can be provided using a syntax very similar to #28598, enabling consistent hashing distribution of keys;
- `failover=error/distribute/slaves` can be set to direct reads at slave servers;
- extra options are passed as is to the driver (e.g. `profile=2.8`)
- Predis per-server settings are also possible, using e.g. `host[localhost][alias]=foo` in the query string, or `host[localhost]=alias%3Dfoo` (ie PHP query arrays or urlencoded key/value pairs)

Commits
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a42e877 [Cache] added support for connecting to Redis clusters via DSN