Check for presence of innodb_large_prefix during plugin activation

[jonathanstegall]

Is your feature request related to a problem? Please describe.
Yes. It's part of #121, but this would serve a specific set of users with that problem.

One thing that came up in this forum post is that hosts might not have innodb_large_prefix enabled on the MySQL server.

Describe the solution you'd like
It would be nice to check for that feature's presence during the activation process. Then we could help those users avoid getting too far into this for no reason.

Describe alternatives you've considered
I will need to investigate whether checking for that feature is even possible.

[jonathanstegall]

Reading more about this, I'm seeing that innodb_large_prefix:

1. Was introduced as an option in MySQL 5.5.
2. Was enabled by default in MySQL 5.7.7.
3. Was also deprecated in MySQL 5.7.7 and will be removed at some point in the future, not because the capability is going away but because newer versions of InnoDB don't need it anymore.

So this is important for users running MySQL versions between 5.5 and 5.7.7.