When growing via push_back(), std::vector may need to reallocate
its internal block of memory due to not enough space. It is advised
to allocate the required space before appending elements if the
size is known beforehand.
This change introduces usage of std::vector::reserve() in api.hh
to ensure that push_back() does not cause reallocations.
Signed-off-by: Patryk Wrobel <patryk.wrobel@scylladb.com>
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