[bugfix] same SMTP configs usage for different stores

[vpodorozh]

Purpose of PR

Fix bugs with the usage of the same transport object (as a result same SMTP configs) for different stores.

Steps to reproduce

1. Prepare two different websites with different SMTP configs;
2. Place two orders on different websites;
3. Execute email sending for those orders in one PHP process.

Expected results

Emails will be sent with corresponding SMTP configs for each website.

Actual result

• First order is sent correctly.
• Second order in a chain will gonna use SMTP configs from website/store of the First order and fail with error:

Failed to send email 554 5.2.252 SendAsDenied ***@***.com; not allowed to send as ***@***.com
STOREDRV Submission Exception SendAsDeniedException Map ...

Solution Description

Introduce store-scoped storage for transport object.
The transport object incapsulates connection configs, connection socket itself and persists connection state.
For different stores, with different configs, connection configs and connection sockets should be different.

[vpodorozh]

Duplicated of #398