Only flush leaf_set if we have a prunable backend

[antiochp]

Currently we have an "empty" leaf_set lying around in each PMMR backend dir for non-prunable backends.
We only use the leaf_set if we have a prunable backend. For non-prunable backends leaves are simply leaves (once added they are never removed).

This PR simply makes makes sure we only sync the leaf_set to disk if the backend is prunable.

Before (note the "empty" unused leaf_set file) -

ls -Gng chain_data/txhashset/kernel
total 184
-rw-r--r--  1 20  53352 Apr  8 14:38 pmmr_data.bin
-rw-r--r--  1 20  29792 Apr  8 14:38 pmmr_hash.bin
-rw-r--r--  1 20      8 Apr  8 14:38 pmmr_leaf.bin

After -

ls -Gng chain_data/txhashset/kernel       
total 176
-rw-r--r--  1 20  53352 Apr  8 14:27 pmmr_data.bin
-rw-r--r--  1 20  29792 Apr  8 14:27 pmmr_hash.bin