Simultaneous calls to a new channel causes errors

[karel-advantitge]

Describe the bug
The AuthGuard checks whether the activeChannel on the session for an incoming request matches the channel of the requestcontext.
If this does not match the session, is updated and the customer performing the request is assigned to the active channel, using the function channelService.assignToChannels().
If a caller performs multiple simultaneous calls to new channel, this logic is executed for each of these calls simultaneaously. This calls the function channelService.assignToChannels() multiple times simultaneously with the same data. The first call succeeds, but the other calls result in an SQL error: ER_DUP_ENTRY: Duplicate entry '6-2' for key 'customer_channels_channel.PRIMARY'

To Reproduce
Steps to reproduce the behavior:

1. Create a customer
2. Perform multiple queries to the shop-api simultaneously with a vendure-token header of a channel the customer is not yet assigned to