[#1302] writecache: Allow to specify custom page size

Signed-off-by: Dmitrii Stepanov <d.stepanov@yadro.com>

docs/storage-node-configuration.md

@@ -290,6 +290,7 @@ writecache:
   small_object_size: 16384
   max_object_size: 134217728
   flush_worker_count: 30
+  page_size: '4k'
 ```
 
 | Parameter            | Type       | Default value | Description                                                                                                                   |
@@ -302,6 +303,7 @@ writecache:
 | `flush_worker_count` | `int`      | `20`          | Amount of background workers that move data from the writecache to the blobstor.                                              |
 | `max_batch_size`     | `int`      | `1000`        | Maximum amount of small object `PUT` operations to perform in a single transaction.                                           |
 | `max_batch_delay`    | `duration` | `10ms`        | Maximum delay before a batch starts.                                                                                          |
+| `page_size`          | `size`     |  `0`          | Page size overrides the default OS page size for small objects storage. Does not affect the existing storage.                 |
 
 
 # `node` section