refactor(ephemeral_file): reuse owned_buffers_io::BufferedWriter (#7484)

part of https://github.com/neondatabase/neon/issues/7124

Changes
-------

This PR replaces the `EphemeralFile::write_blob`-specifc `struct Writer`
with re-use of `owned_buffers_io::write::BufferedWriter`.

Further, it restructures the code to cleanly separate

* the high-level aspect of EphemeralFile's write_blob / read_blk API
* the page-caching aspect
* the aspect of IO
  * performing buffered write IO to an underlying VirtualFile
* serving reads from either the VirtualFile or the buffer if it hasn't
been flushed yet
* the annoying "feature" that reads past the end of the written range
are allowed and expected to return zeroed memory, as long as one remains
within one PAGE_SZ

pageserver/src/virtual_file.rs

@@ -37,7 +37,6 @@ pub(crate) use io_engine::IoEngineKind;
 pub(crate) use metadata::Metadata;
 pub(crate) use open_options::*;
 
-#[cfg_attr(not(target_os = "linux"), allow(dead_code))]
 pub(crate) mod owned_buffers_io {
     //! Abstractions for IO with owned buffers.
     //!