[Don't merge] Use private container to get database connection

[t-ae]

Multiple requests can grab same database connection.
See DatabaseKit issue #43 for details.

Request tries to get database connection from self (as Container). Internally the connection is cached in the container.

But Request is AliasedContainer, so cache is shared among the requests.
If the server receives two (or more) requests simultaneously, they use the same database connection and it can crash here for MySQL, or somewhere else for other DB.

To make cache not shared, I changed to use privateContainer instead of Request itself.
There is similar situation so I think it may work well.

Checklist

• Circle CI is passing (code compiles and passes tests).
• There are no breaking changes to public API.
• New test cases have been added where appropriate.
• All new code has been commented with doc blocks ///.

[t-ae]

Well, it turned out my implementation has a problem.

To make cache not shared, I changed to use privateContainer instead of Request itself to call requestCachedConnection.
But internally, requestCachedConnection tries to get DatabaseConnectionPool. The connection pools are also separated if we use privateContainer.

[t-ae]

Created another PR for DatabaseKit.
vapor/database-kit#44

After that is merged, I'll continue this PR.

[tanner0101]

@t-ae thanks for the great work tracking down this bug. I've merged in your PR to DatabaseKit and tagged as 1.3.0. You should be able to continue on this PR now 👍

[tanner0101]

I went ahead and made the update to DBKit 1.3.0. Thanks again for this PR!

[penny-coin]

Hey @t-ae, you just merged a pull request, have a coin!

You now have 3 coins.

[pankajsoni19]

Package.json still uses db kit 1.0.0. Can we update the supporting dependencies for vapor?