Ensure unicity on primary keys on different calls

[ManuelAlvarezC]

Currently, primary keys are generated using exrex module and the regex from the meta.json file.The way it's implemented, if we sample a single time, we are guaranteed that the primary keys will be unique, however, if we sample more than once, it's possible to obtain again keys that have been returned in the previous call.

Should we ensure uniqueness in this scenario?
Note that if we do this, we will only be able to sample as many rows as different matches the regex allows, afterwards we'll need a way to reset the database before sampling anything else.

For example, if we had a dataset consisting of a single table, with a single column, which is the primary key with regex [1-5]{1}, then the following could happen:

>>>...
>>>first_samples = sampler.sample_all(num_rows=3)
>>>first_samples.T
   primary_key
0            1
1            2
2            3

# Then it's no guaranteed that if we sample a single row more, it's primary key will be neither 4 or 5
>>> second_sample = sampler.sample_all(num_rows=1)
>>> second_sample
   primary_key
0            3