Move SSH command creation off NioProcessors

Creating an SSH command for an incoming user request may require
looking up group information in LDAP if the user's groups are not hot
in the cache. This can take some time and may temporarily block an
NioProcessor thread preventing network IO from occurring for other
active user sessions.

Shift command creation onto a background work queue that only does
command construction for incoming requests. This way active commands
are not blocked by LDAP group lookups.

Two threads are used to try and avoid a single LDAP lookup from
blocking all new command creation on the server.

Change-Id: I1b49a836ba3443a9a85c29b7e3156558ca34ac47
Signed-off-by: Shawn O. Pearce <sop@google.com>
3 files changed