This commit creates a new `Response` interface.
But still, only one `Response` implementation exists:
`ConnResponse`. Custom responses (including mocks)
are expected to implement this interface.

Create a `Response` interface and its implementation
`ConnResponse`.

For the `Future` method `Get` now returns response data.
To get the actual response new method `GetResponse` is added.

`IsPush()` method is added to the response iterator. It returns
the information if the current response is a `Push`.
Right now it does not have a lot of usage, but it will be
crucial once we create a separate response for pushes.

Part of #237

Different implementations of the `Response` interface created.
Special types of responses are used with special requests.

Thus `Response` interface was simplified: some special methods
were moved to the corresponding implementations.
This means that if a user wants to access this methods, the
response should be casted to its actual type.

`SelectResponse`, `ExecuteResponse`, `PrepareResponse`,
`PushResponse` are part of a public API. `Pos()`, `MetaData()`,
`SQLInfo()` methods created for them to get specific info.

`Future` constructors now accept `Request` as their argument.
`Future` methods `AppendPush` and `SetResponse` accept
response `Header` and data as their arguments.

`IsPush()` method is used to return the information if the current
response is a `PushResponse`. `PushCode` constant is removed. To get
information, if the current response is a push response, `IsPush()`
method could be used instead.

After this patch, operations `Ping`, `Select`, `Insert`, `Replace`,
`Delete`, `Update`, `Upsert`, `Call`, `Call16`, `Call17`, `Eval`, `Execute`
of a `Connector` return response data instead of an actual responses.
After this patch, operations `Ping`, `Select`, `Insert`, `Replace`,
`Delete`, `Update`, `Upsert`, `Call`, `Call16`, `Call17`, `Eval`, `Execute`
of a `Pooler` return response data instead of an actual responses.

Part of #237

Create a mock implementations `MockRequest`, `MockResponse` and
`MockDoer`.
The last one allows to mock not the full `Connection`, but its
part -- a structure, that implements new `Doer` interface
(a `Do` function).

Also added missing comments, all the changes are recorded in
the `CHANGELOG` and `README` files. Added new tests and
examples.

So this entity is easier to implement and it is enough to
mock tests that require working `Connection`.
All new mock structs and an example for `MockDoer` usage
are added to the `test_helpers` package.

Added new structs `MockDoer`, `MockRequest` to `test_helpers`.
Renamed `StrangerResponse` to `MockResponse`.

Added new connection log constant: `LogAppendPushFailed`.
It is logged when connection fails to append a push response.

Closes #237

1.10.14 build is flacky on macOS-12 runner.
Bump to 1.10.15 hopefully helps with this issue.

TestConn_ReadWrite causes endless read/write in goroutines after
execution of the test. The patch fixes the problem.

The type Instance:
```
type Instance struct {
	// Name is an unique name of the instance.
	Name   string
	// Dialer will be used to create a connection to the instance.
	Dialer tarantool.Dialer
	// Opts will be used to specify a connection options.
	Opts   tarantool.Opts
}
```

The type allows to specify a dialer and connection options per a
pool instance. It is used in `pool.Connect`, `pool.ConnectWithOpts`
and `pool.Add` to specify an instance configuration now.

Closes #356