Turn BlockCursor::{read_blob,read_blob_into_buf} async fn (#4905)

## Problem

The `BlockCursor::read_blob` and `BlockCursor::read_blob_into_buf`
functions are calling `read_blk` internally, so if we want to make that
function async fn, they need to be async themselves.

## Summary of changes

* We first turn `ValueRef::load` into an async fn.
* Then, we switch the `RwLock` implementation in `InMemoryLayer` to use
the one from `tokio`.
* Last, we convert the `read_blob` and `read_blob_into_buf` functions
into async fn.

In three instances we use `Handle::block_on`:

* one use is in compaction code, which currently isn't async. We put the
entire loop into an `async` block to prevent the potentially hot loop
from doing cross-thread operations.
* one use is in dumping code for `DeltaLayer`. The "proper" way to
address this would be to enable the visit function to take async
closures, but then we'd need to be generic over async fs non async,
which [isn't supported by rust right
now](https://blog.rust-lang.org/inside-rust/2022/07/27/keyword-generics.html).
The other alternative would be to do a first pass where we cache the
data into memory, and only then to dump it.
* the third use is in writing code, inside a loop that copies from one
file to another. It is is synchronous and we'd like to keep it that way
(for now?).

Part of #4743

pageserver/ctl/src/layers.rs

@@ -72,7 +72,7 @@ async fn read_delta_file(path: impl AsRef<Path>) -> Result<()> {
         .await?;
     let cursor = BlockCursor::new(&file);
     for (k, v) in all {
-        let value = cursor.read_blob(v.pos())?;
+        let value = cursor.read_blob(v.pos()).await?;
         println!("key:{} value_len:{}", k, value.len());
     }
     // TODO(chi): special handling for last key?