postgres: add schemas

[t-bast]

Split the table in two different schemas. Trace should do the same: its tables should be in a "trace" schema.

We can then configure roles for schema access. I think this shouldn't be done inside go-core but rather as an OPS script on the DB itself. Here is what I think we should do in such a script:

• Create a user "store" in the DB with a given password
• Create a user "trace" in the DB with a given password
• Alter store schema permissions to allow both store and trace users to access it
• Alter store_private schema permissions to allow only store user to access it
• Alter trace schema permissions to allow only trace user to access it
• Provide the right connection strings so that the store http service connects as the store user and the trace API backend connects as the trace user

Does that make sense?

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This change is 

[codecov[bot]]

Codecov Report

Merging #460 into master will decrease coverage by 0.12%. The diff coverage is 6.97%.

@@            Coverage Diff             @@
##           master     #460      +/-   ##
==========================================
- Coverage   72.81%   72.68%   -0.13%     
==========================================
  Files          86       86              
  Lines        4969     4976       +7     
==========================================
- Hits         3618     3617       -1     
- Misses       1015     1024       +9     
+ Partials      336      335       -1

Impacted Files	Coverage Δ
postgresstore/cmd.go	0% <0%> (ø)	:arrow_up:
postgresstore/postgresstore.go	76.25% <0%> (+1.55%)	:arrow_up:
postgresstore/stmts.go	98.29% <100%> (ø)	:arrow_up:

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