storage: change map type for _db in kvstore

In an effort to move to fragmented data structures, the `absl::btree_map`
type used in the `kvstore` is now replaced by a `chunked_hash_map`.

We do not rely on the ordering or any sort of pointer/reference/iterator
stability provided by `absl::btree_map` in our current uses of `_db`, so
this is a painless swap.

src/v/storage/kvstore.h

@@ -11,7 +11,9 @@
 
 #pragma once
 #include "base/seastarx.h"
+#include "bytes/bytes.h"
 #include "bytes/iobuf.h"
+#include "container/chunked_hash_map.h"
 #include "metrics/metrics.h"
 #include "storage/fwd.h"
 #include "storage/ntp_config.h"
@@ -91,6 +93,8 @@ struct kvstore_config {
 
 class kvstore {
 public:
+    using map_t = chunked_hash_map<bytes, iobuf>;
+
     enum class key_space : int8_t {
         testing = 0,
         consensus = 1,
@@ -164,7 +168,7 @@ class kvstore {
     // Protect _db and _next_offset across asynchronous mutations.
     mutex _db_mut{"kvstore::db_mut"};
     model::offset _next_offset;
-    absl::btree_map<bytes, iobuf, bytes_type_cmp> _db;
+    map_t _db;
     std::optional<ntp_sanitizer_config> _ntp_sanitizer_config;
 
     ss::future<> put(key_space ks, bytes key, std::optional<iobuf> value);