Add PostgreSQL database support

[burleight]

I believe PostgreSQL support is possible by changing the DatabaseStore->write INSERT statement to use ON CONFLICT instead of ON DUPLICATE. Take this subclass of DatabaseStore for example:

use SilverStripe\Core\ClassInfo;
use SilverStripe\Core\Convert;
use SilverStripe\HybridSessions\Store\DatabaseStore;
use SilverStripe\ORM\DB;
use SilverStripe\PostgreSQL\PostgreSQLDatabase;

class PostgreSQLDatabaseStore extends DatabaseStore
{
    protected function isDatabaseReady()
    {
        // Such as during setup of testsession prior to DB connection.
        if (!DB::is_active()) {
            return false;
        }

        // If we have a DB of the wrong type then complain
        if (!(DB::get_conn() instanceof PostgreSQLDatabase)) {
            throw new Exception('PostgreSQLDatabaseStore currently only works with PostgreSQL databases');
        }

        // Prevent freakout during dev/build
        return ClassInfo::hasTable('HybridSessionDataObject');
    }

    public function write($session_id, $session_data)
    {
        if (!$this->isDatabaseReady()) {
            return false;
        }

        $expiry = $this->getNow() + $this->getLifetime();

        DB::query($str = sprintf(
            'INSERT INTO "HybridSessionDataObject" ("SessionID", "Expiry", "Data")
            VALUES (\'%1$s\', %2$u, \'%3$s\')
            ON CONFLICT ("SessionID") DO UPDATE SET "Expiry" = %2$u, "Data" = \'%3$s\'',
            Convert::raw2sql($session_id),
            $expiry,
            Convert::raw2sql(static::binaryDataJsonEncode($session_data))
        ));

        return true;
    }
}

[maxime-rainville]

Honestly, this would be a very low priority for us. If you care about this option, you'll have to submit your own PR.