Add timestamps to "maint time" output

Currently "maint time" will print the amount of time a command took.
Sometimes, though, it's useful to have a timestamp as well -- for
example if one is correlating a gdb log with some other log.

This patch adds a timestamp to the start and end of each command when
this setting is in effect.

This also removes a "//" comment and changes scoped_command_stats to
use DISABLE_COPY_AND_ASSIGN; two minor things I noticed while working
on the patch.

Tested on x86-64 Fedora 29.

gdb/ChangeLog
2019-06-06  Tom Tromey  <tromey@adacore.com>

	* maint.h (class scoped_command_stats): Use
	DISABLE_COPY_AND_ASSIGN.
	<print_time>: New method.
	* maint.c (scoped_command_stats, ~scoped_command_stats): Call
	print_time.
	(scoped_command_stats::print_time): New method.

gdb/testsuite/ChangeLog
2019-06-06  Tom Tromey  <tromey@adacore.com>

	* gdb.base/maint.exp: Expect command started/finished output.