lsm: double sst block size to 8K

This seems to give a cool almost 2x performance gain in the "fillrandom"
db bench. I tried 16K and got another nice 25% gain from the 8K number,
but decided to just keep it at 8K for now.

Author: rockwotj

src/v/lsm/core/internal/options.h

@@ -165,7 +165,7 @@ struct options {
     size_t block_cache_size = default_block_cache_size;
 
     // The size of a single block within an SST file.
-    constexpr static size_t default_sst_block_size = 4_KiB;
+    constexpr static size_t default_sst_block_size = 8_KiB;
     size_t sst_block_size = default_sst_block_size;
 
     // The frequency at which to generate a new bloom filter.

src/v/lsm/lsm.h

@@ -108,7 +108,7 @@ struct options {
     size_t block_cache_size = default_block_cache_size;
 
     // The size of a single block within an SST file.
-    constexpr static size_t default_sst_block_size = 4_KiB;
+    constexpr static size_t default_sst_block_size = 8_KiB;
     size_t sst_block_size = default_sst_block_size;
 
     // The frequency at which to generate a new bloom filter.