Create new smux session for each SOCKS connection

Each SOCKS connection has its own set of snowflakes and broker poll
loop. Since the session manager was tied to a single set of snowflakes,
this resulted in a bug where RedialPacketConn would sometimes try to
pull snowflakes from a previously melted pool. The fix is to maintain
separate smux sessions for each SOCKS connection, tied to its own
snowflake pool.
4 jobs for master in 18 minutes and 23 seconds (queued for 2 minutes and 43 seconds)
Status Job ID Name Coverage
  Test
passed #6648
android

00:08:02

passed #6651
debian-testing

00:04:11

passed #6649
go-1.13

00:01:53

passed #6650
go-1.14

00:04:15