common: LogEntry: add 'channel' field

common: LogEntry: add 'channel' field

We now introduce the concept of 'channel', analogous to syslog
facilities, for log entries.  This will, shortly, allow a LogClient
to send messages to more than just the default syslog facility and log
file, also allowing multiple LogClients and having a way to associate
a LogEntry with its rightful owner.

We also add the same field to the MLogAck message so that a LogClient
waiting on a given ack is able to recognize the MLogAck as its own.

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>