feat(Database::Videos): built-in video cache and support for multiple caching backends

I did this to get rid of Redis compatible DBs and for speed purposes.
This is considered experimental, but everything works fine from
what I have tested.

Here are some benchmarks using the built-in benchmark library of
crystal:

\# built-in release
cache get  19.79M ( 50.54ns) (± 4.12%)  32.0B/op  fastest
cache insert   7.88k (126.86µs) (± 2.20%)  65.5kB/op  fastest
cache get   4.31k (232.11µs) (± 5.50%)  104kB/op  fastest

\# redis release
cache get  22.27k ( 44.90µs) (± 6.40%)  264B/op  fastest
cache insert   4.74k (211.01µs) (± 4.72%)  65.7kB/op  fastest
cache get   2.51k (399.11µs) (±13.15%)  129kB/op  fastest

---

OP/s are way higher, and memory usage per call is lower, so it's a win
win.

src/invidious/config.cr

@@ -217,7 +217,16 @@ class Config
 
   property tokens_server : String = ""
 
-  property video_cache : Bool = true
+  property video_cache : VideoCacheConfig
+
+  class VideoCacheConfig
+    include YAML::Serializable
+
+    property enabled : Bool = true
+    property backend : Int32 = 1
+    # Max quantity of keys that can be held on the LRU cache
+    property lru_max_size : Int32 = 18432 # ~512MB
+  end
 
   {% if flag?(:linux) %}
     property reload_config_automatically : Bool = true
@@ -395,6 +404,16 @@ class Config
       end
     end
 
+    if config.video_cache.enabled
+      if !config.video_cache.backend.in?(0, 1, 2)
+        puts "Config: 'video_cache_storage', can only be:"
+        puts "0 (PostgreSQL)"
+        puts "1 (Redis compatible DB) (Default)"
+        puts "2 (In memory LRU)"
+        exit(1)
+      end
+    end
+
     return config
   end
 end