Use emplace_back to reduce the number of copies and allocations

[wandernauta]

Converting from SQLite's internal representation to the vector-of-maps representation to the nlohmann::json representation will always involve some copying, but this reduces that somewhat by replacing push_back and += with emplace.

Measuring packResultsJsonStr with Valgrind on an example database (22MB) and query (mostly small-ish blobs), this gives:

Before:

198,962 allocs, 198,962 frees, 350,745,047 bytes allocated

After:

84,407 allocs, 84,407 frees, 105,728,530 bytes allocated