This series contains an initial implementation of raft persistency module
that uses `raft` system table as the underlying storage model.
"system.raft" table will be used as a backend storage for implementing
raft persistence module in Scylla. It combines both raft log,
persisted vote and term, and snapshot info.
The table is partitioned by group id, thus allowing multi-raft
operation. The rest of the table structure mirrors the fields of
corresponding core raft structures defined in `raft.hh`, such as
`raft::log_entry`.
The raft table stores the only the latest snapshot id while
the actual snapshot will be available in a separate table
called `system.raft_snapshots`. The schema of `raft_snapshots`
mirrors the fields of `raft::snapshot` structure.
IDL definitions are also added for every raft struct so that we
automatically provide serialization and deserialization facilities
needed both for persistency module and for future RPC implmementation.
The first patch is a side-change needed to provide complete
serialization/deserialization for `bytes_ostream`, which we
need when persisting the raft log in the table (since `data`
is a variant containing `raft::command` (aka `bytes_ostream`)
among others).
`bytes_ostream` was lacking `deserialize` function, which is
added in the patch.
The second patch provides serializer for `lw_shared_ptr<T>`
which will be used for `raft::append_entries`, which has
a field with `std::vector<const lw_shared_ptr<raft::log_entry>>`
type.
There is also a patch to extend `fragmented_temporary_buffer`
with a static function `allocate_to_fit` that allocates an
instance of the fragmented buffer that has a specified size.
Individual fragment size is limited to 128kb.
The patch-set also contains the test suite covering basic
functionality of the persistency module.
* manmanson/raft-api-impl-v11:
raft/sys_table_storage: add basic tests for raft_sys_table_storage
raft: introduce `raft_sys_table_storage` class
utils: add `fragmented_temporary_buffer::allocate_to_fit`
raft: add IDL definitions for raft types
raft: create `system.raft` and `system.raft_snapshots` tables
serializer: add `serializer<lw_shared_ptr<T>>` specialization
serializer: add `deserialize` function overload for `bytes_ostream`
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