feat: Cache results

[nutsalhan87]

Issue #859 — Cache Results.

What is done

Caching query results in selected backend. Only non-write queries and queries not in transaction cached. Query parser enabled required.

Cache configured via config file:

[general.cache]
enabled = true
policy = "cache"
ttl = 300

[general.cache.redis]
url = "redis://localhost:6379"
cache_key_prefix = "pgdog:"

Theoretically multiple backends supported because cache storage defined by trait. But implemented only Redis.

There are two policies:

1. cache — cache all (acceptable to cache) queries by default
2. no_cache (default) — cache only those queries which have cache hint

Hints are:

1. cache [ttl = N] — cache this query with provided ttl, or use default ttl from config
2. force_cache [ttl = N] — force update cache for the query in cache storage
3. no_cache — do not cache query even if there is "cache" policy

These hints can be provided via query comments (pgdog_cache) or connection parameters (pgdog.cache):

/* pgdog_cache: cache ttl=300 */ SELECT * FROM orders;

SET pgdog.cache = 'no_cache';

psql postgresql://postgres:postgres@127.0.0.1:5432/postgres?options=-c%20pgdog.cache%3Dcache

And many more in docs/CACHE.md.

What is not done

1. Tests in any kind: unit, integration, stress

[CLAassistant]

All committers have signed the CLA.