Move a connection to its app's home thread at its first request (017)

With several connection threads, a host of apps had each app's
connections spread over all of them: every thread's round handed each
owner a share, and every turn handed answers back to every thread. The
apps took a third more CPU a request than as processes, with five times
the voluntary switches, and when two apps were busy each waited a
kernel tick at its 99th percentile (4.1 ms, against 1.5 ms as two
processes).

Each app now has a home thread (its place in --apps, modulo the
threads). A connection whose first request names an app at home
elsewhere moves there once, its head read and no ticket taken yet: the
home thread adopts its descriptor and the bytes read, and reads the
request again. Each app's requests then meet its owner from one
thread, as from a process of its own. Two busy apps: 99th percentile
1.4 ms, the same 145k writes a second.

server_apps restarts its apps on three connection threads, asks each
app from fresh connections, and checks that some moved and all were
answered right.

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