ssa.c: Some minor code refactoring into ssa_init_ssa_conn

Signed-off-by: Hal Rosenstock <hal@mellanox.com>
ssa.c: More minor code refactoring into ssa_close_ssa_conn

Signed-off-by: Hal Rosenstock <hal@mellanox.com>
ssa.c: Implemented support for multiple downstream connections

Tested with ACM reconnecting to access and access reconnecting
to plugin/core. Formerly, this wouldn't work but now does.

Due to limited machine configuration (2 right now), still need
to test multiple concurrent connections.

Signed-off-by: Hal Rosenstock <hal@mellanox.com>