Warn user if needed when the process is forked

This change warns the library consumer when the process if forked and
we can't be sure that the library work as expected.

This also add some documentation about forking oslo.messaging Transport
object.

Change-Id: I2938421775aa72866adac198d70214856d45e165
Related-bug: #1330199
Merge "Warn user if needed when the process is forked"