director: Add director_ping_idle/max_timeout setting. director_ping_idle_timeout is used when there's otherwise no input coming from the connection. Changed its default from 10 secs to 30 secs. director_ping_max_timeout is used when the other director keeps sending input, but among it is no PONG reply.

director: Add director_max_parallel_moves/kicks settings The director_max_parallel_moves setting controls the default limit, which can still be increased by explicitly using the doveadm director flush --max-parallel" parameter.

director: Add director_output_buffer_size setting This allows configuring the max buffer size for outgoing connections. Previously it was hardcoded to 10 MB, which wasn't necessarily enough for very busy directors.

director: Make director_consistent_hashing=yes mandatory

director: Support flush socket This allows specifying an URI to execute on user kill. It can be of form exec:/path/to/bin, unix:/path/to/socket or tcp:ip:port The location is sent FLUSH username-hash per killed user. You can execute some action there, and you are expected to return '+\nOK\n' as reply once you are done.

director: Remove director_doveadm_port the flag is obsoleted by inet_listener doveadm name. And listener_get_socket_type_fallback() will fallback to token ring when socket name is missing.

director: Added director_user_kick_delay setting. This replaces the hardcoded 2 seconds delay. This setting specifies how long to wait for after user has been kicked from all directors before letting the user login to the new server. This timeout should be large enough that the user's existing processes in the old backend should be finished.

Changed type of internet port values to in_port_t everywhere. Created special SET_IN_PORT setting type for internet port values. Created net_str2port() for parsing internet port values. Removed several atoi() invocations in the process.

director: Added director_consistent_hashing setting. This should have been the default since the beginning. I didn't thik it was worth the trouble originally because in theory it shouldn't matter much if users jump between multiple backends. But this makes caching behavior worse, especially for systems which are using local cache files.

Added director_username_hash setting to specify what part of the username is hashed. The default is "%u" meaning the full username. Another potentially useful value is "%d" for hashing only the domain (i.e. redirect users from the same domain always to same server so they can safely access each others' shared mailboxes).

director: Don't explicitly use base_dir. We're already chdired there, use relative paths.

director: Added director_doveadm_port for accepting doveadm TCP connections.

Added initial implementation of a director process (for NFS users). There are still some unimplemented features and bugs. Also changing mail server list doesn't yet make sure that other directors won't assign the same user to a different server at the same time. --HG-- branch : HEAD