feat(pageserver): split delta writer automatically determines key range (#8850)

close https://github.com/neondatabase/neon/issues/8838

## Summary of changes

This patch modifies the split delta layer writer to avoid taking
start_key and end_key when creating/finishing the layer writer. The
start_key for the delta layers will be the first key provided to the
layer writer, and the end_key would be the `last_key.next()`. This
simplifies the delta layer writer API.

On that, the layer key hack is removed. Image layers now use the full
key range, and delta layers use the first/last key provided by the user.

---------

Signed-off-by: Alex Chi Z <chi@neon.tech>

libs/pageserver_api/src/key.rs

@@ -263,15 +263,6 @@ impl Key {
         field5: u8::MAX,
         field6: u32::MAX,
     };
-    /// A key slightly smaller than [`Key::MAX`] for use in layer key ranges to avoid them to be confused with L0 layers
-    pub const NON_L0_MAX: Key = Key {
-        field1: u8::MAX,
-        field2: u32::MAX,
-        field3: u32::MAX,
-        field4: u32::MAX,
-        field5: u8::MAX,
-        field6: u32::MAX - 1,
-    };
 
     pub fn from_hex(s: &str) -> Result<Self> {
         if s.len() != 36 {