In svnserve, encapsulate the log_file, log_file_mutex pair into a 
logger_t data structure and prove an small API for it.  Make all
of svnserve use the new, nicer API.

* subversion/svnserve/logger.h
  (): new file
  (logger_t): declare new data strcuture
  (cleanup_logger): new pool cleanup function
  (logger__create,
   logger__log_error,
   logger__write): declare new logger API

* subversion/svnserve/logger.c
  (): new file
  (logger_t): define new data strcuture
  (logger__create,
   logger__log_error,
   logger__write): implement new logger API

* subversion/svnserve/server.h
  (log_error): drop
  (server_baton_t,
   serve_params_t): use new logger object instead of plain files

* subversion/svnserve/serve.c
  (log_write,
   log_error_internal,
   log_server_error): drop
  (log_error): greatly simplify
  (error_create_and_log,
   log_fail_and_flush,
   log_command,
   log_authz_denied,
   handle_config_error,
   lookup_access,
   must_have_access,
   add_lock_tokens,
   unlock_paths,
   lock_many,
   unlock_many,
   fs_warning_func,
   serve): update to use the new logger API

* subversion/svnserve/svnserve.c
  (serve_socket): use new logger API
  (main): ditto; update log init code


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1530739 13f79535-47bb-0310-9956-ffa450edef68