msg: rename ms_tcp_read_timeout to ms_connection_idle_timeout

msg: rename ms_tcp_read_timeout to ms_connection_idle_timeout

The old naming is confusing, e.g., it actually indicates we should tear
down the underlying connection which has no read/write activities
at both sides (namely connection is idle) for over 15 minutes.

Signed-off-by: xie xingguo <xie.xingguo@zte.com.cn>
(cherry picked from commit 1d464221d998376561c102c83d3127e646815bbb)

Conflicts:
	src/common/legacy_config_opts.h : Resolved for ms_connection_idle_timeout
	src/common/options.cc : Resolved for ms_connection_idle_timeout
	src/msg/simple/Pipe.cc : rename ms_tcp_read_timeout to ms_connection_idle_timeout