Fix consistency for job id generation within codebase.

[mariacarmina]

Fixes #935 .

Changes proposed in this PR:

• make generateUniqueId util function for generating job ids
• startCompute can receive jobId as optional parameter, because paid start compute handler generates before a job id that can be used and not re-generated, in case this parameter jobId is not provided, docker compute engine will generate a new one based on generateUniqueId output
• update parameters for unit tests when inserting new compute job

[giurgiur99]

Don't we need to add the jobId in FreeComputeStartHandler too? And we can make it non-optional.

[mariacarmina]

Don't we need to add the jobId in FreeComputeStartHandler too? And we can make it non-optional.

We can generate the jobId within free start compute handler and remove the generation from engine.startComputeJob

[giurgiur99]

LGTM!