Original IDL generated code was hardcoded to always use bytes_ostream.
This patch makes the output stream a template parameter so that any
valid output stream can be used.
Unfortunately, making IDL writers generic requires updates in the code
that uses them, this is fixed in C++17 which would be able to deduce the
parameter in most cases.
Schema is tracked in memtable and cache per-entry. Entries are
upgraded lazily on access. Incoming mutations are upgraded to table's
current schema on given shard.
Mutating nodes need to keep schema_ptr alive in case schema version is
requested by target node.
frozen_schema will transfer schema definition across nodes with schema
mutations. Because different nodes may have different versions of
schema tables, we cannot use frozen_mutations to transfer these
because frozen_mutation can only be read using the same version of the
schema it was frozen with. To solve this problem, new from of mutation
is introduced called canonical_mutation, which can be read using any
version of the schema.
