Fix Retry-After handling

[mhoffrog]

Description

Please refer to #4543 for details.

1. We must perform an "inline" sleep on status 503 only
2. Take the Retry-After value in a more robust way by grepping for the value format in seconds only

This PR closes #4543

Test results

Test data

The test has been run offline by performing the grep command on the following file retry-after_test.txt:

# Test case#1: Upper case chars, space before and after :colon:
Retry-After : 12345

# Test case#2: lower case chars, no space before and nor after :colon:, spaces after the value
retry-after:22345           

# Test case#3: Upper case chars, no space before, space after :colon:, no spaces after value
Retry-After: 32345

# Test case#4: Upper case chars, space before and after :colon:, spaces after value
Retry-After : 42345    

# Test case#5: Upper case chars, no space before, space after :colon:, spaces after value
Retry-after: 52345    

# Test case#6: Lower case chars, HTTP date format for value
retry-after : Sun, 21 Oct 2018 12:16:24 GMT

# Test case#7: Upper case chars, HTTP date format for value
Retry-After : Sun, 21 Oct 2018 12:16:24 GMT

Test run (shell command)

$ grep -i "^Retry-After *: *[0-9]\+ *" retry-after_test.txt | cut -d : -f 2 | tr -d ' ' | tr -d '\r'
12345
22345
32345
42345
52345