[orc-rt] Rename "managed code" to "keepalive". NFC. (#216670)
Renames the Session's managed-code task group and its helpers so that
they name the mechanism -- the barrier that holds the Session open --
rather than a category of code:
ManagedCodeTaskGroup -> KeepaliveTaskGroup
Session::callManagedCode -> Session::callWithKeepalive
Session::managedCodeTokenSource -> Session::keepaliveTokenSource
waitForManagedCodeTasksThenShutdown -> waitForKeepalivesThenShutdown
ManagedCodeCaller -> KeepaliveCaller
and retitles the corresponding section of docs/Design.md from "Managed
code execution and shutdown" to "Keepalives and shutdown".
"Managed code" collides with the established .NET sense of the term,
which risks confusion. Naming the mechanism instead makes the purpose
clear at the call site: callWithKeepalive says that the Session is held
open across the call.
Where the documentation still needs a noun for the code itself it now
says "Session-managed code", which keeps the original meaning (JIT'd
code, plus library code loaded on its behalf) without the bare term's
collision.
TaskGroup itself is unchanged and remains generic: a keepalive is the
Session's use of a token, not a new kind of token.