In situations when created connections are used to only send/receive 1
request/response, the overhead to create/shutdown full connections could be
too much. Unfortunately this is exactly the mode that is currently
primarily used for compatibility with NEO/py. To help mitigate the overhead
in such scenarios, lightweight connections mode is provided:

At requester side, one message can be sent over node link with link.Send1 .
Inside a connection will be created and then shut down, but since the
code manages whole process internally and does not show the connection to
user, it can optimize those operations significantly. Similarly link.Ask1
sends 1 request, receives 1 response, and then puts the connection back into
pool for later reuse.

At receiver side, link.Recv1 accepts a connection with the first message
remote peer sent us when establishing it, and wraps the result into Request
object. The Request contains the message received and internally the
connection. A response can be sent back via Request.Reply. Then once
Request.Close is called the connection object that was accepted is
immediately put back into pool for later reuse.

Some history of lightweight mode:

lab.nexedi.com/kirr/neo/commit/0fa96338	X Clarified Request.Close semantics - tests working again
lab.nexedi.com/kirr/neo/commit/a5ac1652	X Ask1: switch to sending directly over link
lab.nexedi.com/kirr/neo/commit/755e3654	X Request.Reply: switch to replying directly over link
lab.nexedi.com/kirr/neo/commit/c643ba53	X Send1: switch to sending directly over link
lab.nexedi.com/kirr/neo/commit/7dcbc9c5	X Send1: switch to lightClose
lab.nexedi.com/kirr/neo/commit/851864a9	X chan RTT benchmark which simulates Recv1 = Accept + Recv
lab.nexedi.com/kirr/neo/commit/099bfc29	X freelist for PktBuf
lab.nexedi.com/kirr/neo/commit/58c2e39a	X Benchmark for link Ask1/Recv1 over TCP loopback