ovsdb-data: Deduplicate string atoms.

ovsdb-server spends a lot of time cloning atoms for various reasons,
e.g. to create a diff of two rows or to clone a row to the transaction.
All atoms, except for strings, contains a simple value that could be
copied in efficient way, but duplicating strings every time has a
significant performance impact.

Introducing a new reference-counted structure 'ovsdb_atom_string'
that allows to not copy strings every time, but just increase a
reference counter.

This change allows to increase transaction throughput in benchmarks
up to 2x for standalone databases and 3x for clustered databases, i.e.
number of transactions that ovsdb-server can handle per second.
It also noticeably reduces memory consumption of ovsdb-server.

Next step will be to consolidate this structure with json strings,
so we will not need to duplicate strings while converting database
objects to json and back.

Signed-off-by: Ilya Maximets <i.maximets@ovn.org>
Acked-by: Dumitru Ceara <dceara@redhat.com>
Acked-by: Mark D. Gray <mark.d.gray@redhat.com>

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