nessDB v1.8.1 using LSM-Tree storage engine

[BohuTANG]

Hi,forkers

nessDB v1.8.1 is using LSM-Tree storage engine now.But I am sorry do
this on the main branch,so some forkers should refork again.
The new comes with the following changes:

nessDB
------------
* enhanced random-write and random-read performance(very fast now)
* using LSM-Tree as storage engine, layout as follow:
* mtable: a memory cache for all operations(such as
add,delete,update),key in which is sorted.
* log: a disk-back log file,for data recovery.
* sst: on-disk index file,which contains SST_BLOCK and a
FOOTER.

Thanks

--BohuTANG