allow DB to use a connection pool.

allow Driver to parse connection pool options for extensibility.
fix waiting_resource counter after a timeout was generated.

spec/db_spec.cr

@@ -61,7 +61,7 @@ describe DB do
   it "should raise if the sole connection is been used" do
     with_dummy do |db|
       db.query "1" do |rs|
-        expect_raises Exception, /DB Pool Exhausted/ do
+        expect_raises DB::PoolTimeout do
           db.scalar "2"
         end
       end

spec/dummy_driver.cr

@@ -187,7 +187,7 @@ end
 def with_dummy
   DummyDriver::DummyConnection.clear_connections
 
-  DB.open "dummy://host" do |db|
+  DB.open "dummy://host?checkout_timeout=0.5" do |db|
     yield db
   end
 end

spec/pool_spec.cr

@@ -131,6 +131,13 @@ describe DB::Pool do
     end
   end
 
+  it "should be able to release after a timeout" do
+    pool = DB::Pool.new(->{ Closable.new }, max_pool_size: 1, checkout_timeout: 0.1)
+    a = pool.checkout
+    pool.checkout rescue nil
+    pool.release a
+  end
+
   it "should close if max idle amount is reached" do
     all = [] of Closable
     pool = DB::Pool.new(->{ Closable.new.tap { |c| all << c } }, max_pool_size: 3, max_idle_pool_size: 1)