Introduce new storage component Peapod

Currently, storage node saves relatively small NeoFS objects in
Blobovnicza tree component: group of BoltDB wrappers managed as a tree.
This component has pretty complex data structure, code implementation
and dubious performance results.

Peapod is a new storage component introduced to replace Blobovnicza one
as more simple and effective. It also bases on single BoltDB instance,
but organizes batch writes in a specific way. In future, Peapod is going
to be used as a storage of small objects by the BlobStor.

Refs nspcc-dev#2453.

Signed-off-by: Leonard Lyubich <[email protected]>