Improve multistatement support for postgres

[nobbynobbs]

Hi!

I opened this PR because:

multistmt is naive and simply splits by ; so it's incorrectly separating the body of your function

Before open in it, I've found related issue and the couple of staled PR's:

• pq: unterminated dollar-quoted string at or near "$BODY$ #590 - just issue describes the problem
• Allow multi-statement mode to be enabled per-migration in Postgres #693 - suggests to use meta-language in comments and looks rejected because of "violates the project's principle around the migration content format"
• feat: replace mulit statement parsing with comment aware parsing #691 - PR about similar, but different problem

In this PR I suggest to complicate Postgres multi-statement parser, but do not implement all the lexer/parser machinery, preserve the same approach that already used, but make it be respectful to dollar quoted string literals.

Also, I've tried another approach, like use third-party library to parse migration and split it to statements. Sounds good, implementation looks pretty, but doesn't work at all, see nobbynobbs#1

[coveralls]

coverage: 59.56% (+0.4%) from 59.21%
when pulling d849290 on nobbynobbs:improve-postgres-multistatement
into 0815e2d on golang-migrate:master.

[bnkamalesh]

just hoping/looking forward to getting this merged asap!

[bnkamalesh]

hello @dhui would this PR be considered?