pull_timeline and sk logging fixes (#11786)

This patch contains some fixes of issues I ran into for #11712:

* make `pull_timeline` return success for timeline that already exists.
This follows general API design of storage components: API endpoints are
retryable and converge to a status code, instead of starting to error.
We change the `pull_timeline`'s return type a little bit, because we
might not actually have a source sk to pull from. Note that the fix is
not enough, there is still a race when two `pull_timeline` instances
happen in parallel: we might try to enter both pulled timelines at the
same time. That can be fixed later.
* make `pull_timeline` support one safekeeper being down. In general, if
one safekeeper is down, that's not a problem. the added comment explains
a potential situation (found in the `test_lagging_sk` test for example)
* don't log very long errors when computes try to connect to safekeepers
that don't have the timeline yet, if `allow_timeline_creation` is false.
That flag is enabled when a sk connection string with generation numbers
is passed to the compute, so we'll hit this code path more often. E.g.
when a safekeeper missed a timeline creation, but the compute connects
to it first before the `pull_timeline` gets requested by the storcon
reconciler: this is a perfectly normal situation. So don't log the whole
error backtrace, and don't log it on the error log level, but only on
info.

part of #11670

libs/postgres_backend/src/lib.rs

@@ -841,6 +841,10 @@ impl<IO: AsyncRead + AsyncWrite + Unpin> PostgresBackend<IO> {
 
         let expected_end = match &end {
             ServerInitiated(_) | CopyDone | CopyFail | Terminate | EOF | Cancelled => true,
+            // The timeline doesn't exist and we have been requested to not auto-create it.
+            // Compute requests for timelines that haven't been created yet
+            // might reach us before the storcon request to create those timelines.
+            TimelineNoCreate => true,
             CopyStreamHandlerEnd::Disconnected(ConnectionError::Io(io_error))
                 if is_expected_io_error(io_error) =>
             {
@@ -1059,6 +1063,8 @@ pub enum CopyStreamHandlerEnd {
     Terminate,
     #[error("EOF on COPY stream")]
     EOF,
+    #[error("timeline not found, and allow_timeline_creation is false")]
+    TimelineNoCreate,
     /// The connection was lost
     #[error("connection error: {0}")]
     Disconnected(#[from] ConnectionError),