Fix race condition by capturing version in goroutine

[klim0v]

The problem is related to a race condition when using the versionToUse variable in concurrent goroutines:

Problem Description

At line 246 (versionToUse = GetPayloadV1), a local variable versionToUse is modified, which is captured by the closure of all running goroutines (lines 159-314). This leads to the following issue:

1. N goroutines are launched for N relays with versionToUse == GetPayloadV2
2. Relay A doesn't support V2 API → returns an error (StatusCode >= 400)
3. Goroutine A modifies the shared variable to versionToUse = GetPayloadV1 (line 246)
4. Relay B supports V2 API → returns a valid response with StatusCode == 202 (Accepted)
5. Goroutine B tries to process the response, but due to the variable change, enters the condition if versionToUse == GetPayloadV1 (line 266)
6. Parsing of a valid V2 response occurs as V1, leading to a decoding error

Technical Details

All goroutines share a single versionToUse variable captured through closure. When one goroutine modifies its value (line 246), it immediately affects the processing logic in other goroutines (lines 266-302), which may be at different execution stages.

Consequences

• Valid V2 responses will be parsed as V1
• A decoding error will occur (line 288)
• Valid payload will be rejected
• Potentially may lead to a missed slot