add prepared_statements option to database

* use ?prepared_statements=true|false on connection string (default: true)
* make inmutable state in database
* make mutable state in connection
* change Connection#build to use the current prepared_statements flag to build prepared or unprepared statements.

src/db.cr

@@ -120,6 +120,15 @@ module DB
   private def self.build_database(uri : URI)
     Database.new(driver_class(uri.scheme).new, uri)
   end
+
+  # :nodoc:
+  def self.fetch_bool(params : HTTP::Params, name, default : Bool)
+    if value = params[name]?
+      value.underscore == "true"
+    else
+      default
+    end
+  end
 end
 
 require "./db/pool"

src/db/connection.cr

@@ -24,15 +24,19 @@ module DB
     # :nodoc:
     getter database
     @statements_cache = StringKeyCache(Statement).new
+    property? prepared_statements : Bool
 
     def initialize(@database : Database)
+      @prepared_statements = @database.prepared_statements?
     end
 
     # :nodoc:
     def build(query) : Statement
-      # TODO add flag for default statements kind.
-      # configured in database overridable by connection
-      fetch_or_build_prepared_statement(query)
+      if prepared_statements?
+        fetch_or_build_prepared_statement(query)
+      else
+        build_unprepared_statement(query)
+      end
     end
 
     # :nodoc:

src/db/database.cr

@@ -25,6 +25,8 @@ module DB
     # Returns the uri with the connection settings to the database
     getter uri
 
+    getter? prepared_statements : Bool
+
     @pool : Pool(Connection)
     @setup_connection : Connection -> Nil
     @statements_cache = StringKeyCache(PoolStatement).new
@@ -32,6 +34,7 @@ module DB
     # :nodoc:
     def initialize(@driver : Driver, @uri : URI)
       params = HTTP::Params.parse(uri.query || "")
+      @prepared_statements = DB.fetch_bool(params, "prepared_statements", true)
       pool_options = @driver.connection_pool_options(params)
 
       @setup_connection = ->(conn : Connection) {}