feat(embedded/tools): initial SQL stress tool

[dmacvicar]

This is an initial stress tool following the same pattern as stress_tool.go but using the embedded SQL engine.

Caveats:

• It fails if the database exists
• It generates serial IDs without persisting the last, therefore 2 consecutive runs would fail with duplicate IDs (perhaps a flag in the future indicating starting ID would help)

[coveralls]

Coverage increased (+0.05%) to 87.646% when pulling 845682b on feat/sql_stress_tool into 3fb881f on master.

[SimoneLazzaris]

Looks good for a first implementation. Possible improvements:

• add an option to use an existing database/table, so that the test can be run in series
• give the option to read the values instead of just doing insert, so that we can also test read speed
• git the option to have sequences, so that we can also check procedurally if insert are correctly read back