feat(logger): configure syslog and file logging (#162)

Allows optional configuration to start up file or syslog logging.  File logging may include file rotation and/or watching.

Rotating file logging is necessary for running on cedar.  Log messages emitted before enabling the log file are buffered and flushed to the log file once it is opened, meaning the config-loading code can still use the log even before a log-file is opened.

Moves the initial logging setup from import time to an explicit call to `init_logging`.

alpenhorn/config.py

@@ -31,16 +31,78 @@
         - alpenhorn_chime
         - chimedb.core.alpenhorn
 
-    # Logging configuration
+    # Logging configuration.  By default, alpenhorn sends all log message to
+    # standard error.
     logging:
-
         # Set the overall logging level
         level: debug
 
         # Allow overriding the level on a module by module basis
         module_levels:
             alpenhorn.db: info
 
+        # Alpenhorn can be configured to send log output to syslog and/or
+        # a file.  This is _in addition_ to the log sent to standard error,
+        # which is always enabled.
+
+        # Syslog logging.
+        syslog:
+            # If true, enable logging to syslog.  Note: if the "syslog" section
+            # is present in the logging config, then the default value
+            # of this key is true.  As a result, this key need only be specified if
+            # none of the other syslog configuration parameters are provided
+            # in the config.
+            enable: true
+
+            # The network address to send syslog message to.
+            #
+            # May also be a Unix domain socket (like "/dev/log").  In that case,
+            # set `port`, below, to zero to indicate this.
+            #
+            # Defaults to "localhost".
+            address: localhost
+
+            # The network port to send syslog messages to.  If this is zero,
+            # then the port is ignored and `address` is taken to be a Unix domain
+            # socket.  Defaults to 514, the standard syslog port.
+            port: 514
+
+            # The syslog facility to use.  If given, should be the name of one of
+            # the syslog facilities, disregarding case ("user", "local0", ...)
+            # Defaults to "user".
+            facility: user
+
+            # Set to True to use TCP, instead of UDP, to send messages to the syslog server.
+            # Ignored if using a Unix domain socket (i.e. `port` is zero).  Default is False.
+            use_tcp: false
+
+        # File logging.
+        file:
+            name: /path/to/file.log
+
+            # If a third-party (like logrotate) is rotating the alpenhorn log
+            # file, set this to "true" to tell alpenhorn to watch for log-file
+            # rotation.
+            watch: false
+
+            # Alternately, alpenhorn can manage log file rotation itself.  Set
+            # "rotate" to true to enable.  At most one of "watch" and "rotate"
+            # may be true.
+            rotate: true
+
+            # The following two settings affect alpenhorn-managed log rotation
+            # and are ignored if "rotate" is not true.
+
+            # Maximum number of rotated files to keep.  Must be at least one
+            # Rotated files append an integer to the name specified (e.g.
+            # "file.log.1", "file.log.2" etc.).
+            backup_count: 100
+
+            # Size, in bytes, at which log file rotation occurs.  May include a suffix:
+            # k, M, or G.
+            max_bytes: 4G
+
+
     # Configure the operation of the local service
     service:
         # Default number of worker threads