buffer module: support arbitrary ev_loops

Change buffer module interface to make it more flexible due to ev_now()
call introduced in c7a3cdece9.
Use PACKAGE_NAME rather than "sniproxy" string literal
Share file handles between loggers to same file

Split out LogSink for Logger, use single LogSink for all syslog Loggers
and only open syslog socket once.
Format timestamp only once per second