sd-dhcp-server: set DHCPLease::server before hashmap_put()

sd-dhcp-server: set DHCPLease::server before hashmap_put()

Otherwise, if the second push is failed, then the first hashmap contains
dirty entry.

Also, this makes hashmap_remove_value() used when removing leases to
make not wrong lease is removed from the hashmap.

Note, this just hide the root cause of the issue #22253, which will be
fixed in later commit.

Fixes #22253.

(cherry picked from commit 8a7d048d1ddb8916482f1422405d3e0e4bccb279)