storage: improve SST builder with FullKey struct

[hzxa21]

#6130 introduced a struct for FullKey (#5960):

pub struct TableKey<T: AsRef<[u8]>>(pub T);

pub struct UserKey<T: AsRef<[u8]>> {
    pub table_id: TableId,
    pub table_key: TableKey<T>,
}

pub struct FullKey<T: AsRef<[u8]>> {
    pub user_key: UserKey<T>,
    pub epoch: HummockEpoch,
}

This makes the storage interface clearer to avoid misusage and saves some space by delaying storing shared part (e.g. table_id, epoch) repeatedly in the immutable memory buffer.

However, several improvements can also be done inside SST builder:

• Let filter key extractor be aware of UserKey struct to avoid unnecessary key copy. We can further maintain per table bloom filter inside a SST to improve efficiency of the bloom filter potentially.
• Let prefix compression be aware of FullKey struct to avoid unneccessary key copy and allow better compression potentially.
• minor: let block builder be aware of FullKey struct and append different parts of the key into the block builder buffer sequentially to save unneccessary memory allocation for encoding full key