feat: refactor stats method in wal

[Rachelint]

Rationale

Although get_statistic method exist in WalManager, it print so less message and just impl in RocksDB wal.
In this pr, I impl this method in all wal impls, and print the detail inner infos through it.

The output example(RocksDB wal):

[Data wal stats]:
#RocksDB stats:
#Wal stats:
TableUnit { id: 2199023255724, next_sequence_num: 59 }
TableUnit { id: 2199023255777, next_sequence_num: 50 }
TableUnit { id: 2199023255627, next_sequence_num: 50 }
...

------------------------------------------------------

[Manifest wal stats]:
#RocksDB stats:
#Wal stats:
TableUnit { id: 2199023255739, next_sequence_num: 2 }
TableUnit { id: 2199023255669, next_sequence_num: 2 }
TableUnit { id: 2199023255740, next_sequence_num: 2 }
...

Detailed Changes

Impl and improve the get_statistic method in WalManager.

Test Plan

Test manually.

[ShiKaiWi]

LGTM