Support for sending connection attributes

[Vasfed]

Description

This PR adds support for sending connection attributes, which are used for identifying individual connections in mysql. (https://dev.mysql.com/doc/refman/5.7/en/performance-schema-connection-attribute-tables.html)
For example libmysql sets attributes _client_name, _pid, _client_version, _os, _platform. This adds default _client_name=go-mysql-driver

Checklist

• [+] Code compiles correctly
• [+] Created tests which fail without the change (if possible)
• [+] All tests passing
• [+] Extended the README / documentation, if necessary
• [+] Added myself / the copyright holder to the AUTHORS file

[dolmen]

• This feature requires the performance_schema to be enabled on the server, so this should be clearly documented with the option.
• Rename the option to connectAttrs as this feature is named connect_attrs in MySQL. Also ConnectAttrs instead of Attributes in struct Config.
• TestConnectionAttributes should use table INFORMATION_SCHEMA.PROCESSLIST instead of sys.processlist if possible because the sys schema is introduced only in MySQL 5.7.7.

[Vasfed]

Removed sys.processlist from test, renamed option to connectAttrs.
Cannot get tests to pass on travis yet (connection seems not to appear in performance_schema in 5.6, trying to reproduce)

[Vasfed]

@dolmen fixed your suggestions.
Tests were failing on travis because default mysql 5.6 in ci-garnet image has performance_schema=OFF

[Vasfed]

Rebased onto current master to resolve merge conflicts

[dolmen]

Commit "Fix excessive null-termination for auth data in handshake" do not seem related to this PR. Or is it?

[Vasfed]

@dolmen strictly speaking - it is not, and can be merged alone, but it was included because until it is fixed this PR cannot be merged.
Will look into conflicts - may be it is already fixed in master over all this time

[dolmen]

It would be useful to add tests and make them run under Travis-CI (see file .travis.yml).

Here is a scenario:

• create a DSN with a program_name
• connect
• check SELECT * FROM performance_schema.session_account_connect_attrs that we get values from the DSN.

[andygrunwald]

Hey @dolmen and @Vasfed,
anything I can help with to get this going?

I was aiming to send the connection attribute program_name with my service.

[andygrunwald]

Hey @dolmen and @Vasfed,
I used this PR, re-integrated it with master and opened a new one: #1241