v2: improve pool.Connect interface

[oleg-jukovec]

The current interface:

func Connect(ctx context.Context, dialers map[string]tarantool.Dialer, connOpts tarantool.Opts) (*ConnectionPool, error)

It has two disadvantages:

1. It does not allow to use different connect options for different connections.
2. It could be tricky to initialize a map of dialers.

I suggest a small refactoring:

1. Add the Server type to the pool package, which would describe one server for a pool:

   type Server struct {
   Id     string
   Dialer tarantool.Dialer
   Opts   tarantool.Opts
   }

2. Update the pool.Connect interface:

   func Connect(ctx context.Context, servers ...Server) (*ConnectionPool, error)

3. Update pool.ConnectWithOpts:

   func ConnectWithOpts(ctx context.Context, opts Opts, servers ...Server) (*ConnectionPool, error)

4. Update pool.Add:

   func (p *ConnectionPool) Add(ctx context.Context, server Server) error