Preserve caller context in TcpAcceptor (#1087)

Preserve caller context in TcpAcceptor (#1087)

Before this fix, acceptOne() was losing FTP DATA TcpAcceptor context
by unconditionally resetting the context to a nil listenPort_.

Also, TcpAcceptor methods should not (be expected to) explicitly set
their job code context. Instead, the job should be created in the
right code context, allowing job-created async calls to auto-manage code
context. The updated port-iterating clientHttpConnectionsOpen() and
Ftp::StartListening() loops now set the right code context when creating
async calls that result in TcpAcceptor job creation.