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space: optimize lookup by id

In contrast to space_cache_find(), space_by_id() doesn't cache the last
looked up space to speed up following lookups. This is confusing,
because one would expect the only difference between space_cache_find()
and space_by_id() functions to be that the former sets diag while the
latter doesn't. Let's move the prev_space cache to space_by_id() to fix
this issue.

To achieve that we need to make space_by_id() inline, which is tricky
because it's called via FFI. We work around that by defining a macro
space_by_id() which expands to space_by_id_fast() while still keeping
the space_by_id() symbol for FFI.

Last but not least, we make the last cached space global (currently it's
instantiated once per each space_cache_find() call site). This is a good
thing because we often call space_by_id() a few times while processing
the same DML request (e.g. in memtx mvcc).

NO_DOC=no user-visible behavior changes
NO_TEST=no user-visible behavior changes
NO_CHANGELOG=no user-visible behavior changes

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