Initial Fjall metadata implementation improvement

[iwanbk]

Fjall metada support was added at #16.
Further improvements still needed:

• assess and add transasction support (like in the sled implementation) fjall transaction #39
• simplify the API: some API still return Object or Block. The metada shouldn't be aware about this. Always do zerocopy for this.
• add proper test Add unit test to the fjall meta store #42
• fine tune the fjall configuration, we still use the default conf

Another improvements which doesn't classify as initial implementation shouldn't be included here, such as #10 and #40