chore: Use smaller merge buffers. (#74)

## What

Reduce the per-segment buffer sizes from 4MB to 512KB.

## Why

#71 moved from buffers which covered the entire file to maximum 4MB buffers. But for merges with very large segment counts, we need to be using more conservative buffer sizes. 512KB will still eliminate most posting list reads: posting lists larger than 512KB will skip the buffer.

Author: stuhood

common/src/buffered_file_slice.rs

@@ -6,16 +6,16 @@ use std::ops::Range;
 use super::file_slice::FileSlice;
 use super::{HasLen, OwnedBytes};
 
-const DEFAULT_BUFFER_MAX_SIZE: usize = 4 * 1024 * 1024; // 4 MB
+const DEFAULT_BUFFER_MAX_SIZE: usize = 512 * 1024; // 512K
 
 /// A buffered reader for a FileSlice.
 ///
 /// Reads the underlying `FileSlice` in large, sequential chunks to amortize
 /// the cost of `read_bytes` calls, while keeping peak memory usage under control.
 ///
 /// TODO: Rather than wrapping a `FileSlice` in buffering, it will usually be better to adjust a
-/// `FileHandle` to directly handle buffering itself (as that allows separate `FileSlice`s read
-/// from the same `FileHandle` to share buffers.)
+/// `FileHandle` to directly handle buffering itself.
+/// TODO: See: https://github.com/paradedb/paradedb/issues/3374
 pub struct BufferedFileSlice {
     file_slice: FileSlice,
     buffer: RefCell<OwnedBytes>,