* Added a sqlXPassThrough layer
This layer embeds the original sqlx object and wraps some
key methods to avoid some boilerplate.
- Add timeout to all methods from the timeout in settings.
- Added trace functionality.
- Added auto-rebind capabilities so that each store method
doesn't need to call rebind every time.
- Added auto-lowering of query string to support NamedExec
for both MySQL and Postgres
All methods are not wrapped as all are likely not to be called
from our code. As and when they are called, support will be added.
```release-note
NONE
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* fix lint issues
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NONE
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* address review comments
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* incorporate suggestions
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NONE
```
This PR builds up on the pass-through DB driver to a fully functioning DB driver implementation via our RPC layer.
To keep things separate from the plugin RPC API, and have the ability to move fast with changes, a separate field Driver is added to MattermostPlugin. Typically the field which is required to be compatible are the API and Helpers. It would be well-documented that Driver is purely for internal use by Mattermost plugins.
A new Driver interface was created which would have a client and server implementation. Every object (connection, statement, etc.) is created and added to a map on the server side. On the client side, the wrapper structs hold the object id, and communicate via the RPC API using this id.
When the server gets the object id, it picks up the appropriate object from its map and performs the operation, and sends back the data.
Some things that need to be handled are errors. Typical error types like pq.Error and mysql.MySQLError are registered with encoding/gob. But for error variables like sql.ErrNoRows, a special integer is encoded with the ErrorString struct. And on the cilent side, the integer is checked, and the appropriate error variable is returned.
Some pending things:
- Context support. This is tricky. Since context.Context is an interface, it's not possible to marshal it. We have to find a way to get the timeout value from the context and pass it.
- RowsColumnScanType(rowsID string, index int) reflect.Type API. Again, reflect.Type is an interface.
- Master/Replica API support.
