fix: create the connection as a promise with asPromise

[jmcdo29]

PR Checklist

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• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[X] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Other... Please describe:

What is the current behavior?

Connections injected with @InjectConnection() might not be initialised

Issue Number: #1082

What is the new behavior?

Connections injected with @InjectConnection() will be ready to use

Does this PR introduce a breaking change?

[ ] Yes
[X] No

I've tested this by modifying the JS in the node_modules and it does work as expected

[kamilmysliwiec]

LGTM