BUG/MEDIUM: server/cli: Fix ABBA deadlock when fqdn is set from the CLI

BUG/MEDIUM: server/cli: Fix ABBA deadlock when fqdn is set from the CLI

To perform servers resolution, the resolver's lock is first acquired then
the server's lock when necessary. However, when the fqdn is set via the CLI,
the opposite is performed. So, it is possible to experience an ABBA
deadlock.

To fix this bug, the server's lock is acquired and released for each
subcommand of "set server" with an exception when the fqdn is set. The
resolver's lock is first acquired. Of course, this means we must be sure to
have a resolver to lock.

This patch must be backported as far as 1.8.