Applying many interfaces at once results in 504 gateway timeout

[jaredhendrickson13]

Describe the bug
When applying many interfaces at once, the interface reload process takes longer than allowed by nginx/PHP. This forces a 504 gateway timeout to be returned.

Expected behavior
Instead of the API waiting for all interfaces to be applied in sequence, clients should be able to request the interfaces to be applied in the background and poll the apply status.

pfSense Version & Package Version:

• pfSense Version: v2.6.0 (affects all versions however)
• Package Version v1.4.2

Affected Endpoints:

• URL: /api/v1/interface/apply