refactor: move skip methods to abstract parser

[olavloite]

Move the PostgreSQL skip methods from the PostgreSQL parser to the abstract parser. This is step 1 in refactoring the GoogleSQL and PostgreSQL parser so they can share more code. The eventual goal is to allow the GoogleSQL parser to be able to handle SQL string without having to remove the comments from the string first.

This pull request only moves some methods from the class PostgreSQLStatementParser to AbstractStatementParser.

This pull request is the first in a series to refactor these statement parsers. The goals of this refactoring is:

1. Share more code between the two parsers (so move more into the abstract base class that is shared between them).
2. The SpannerStatementParser currently requires comments to be removed from the SQL string in order to convert positional parameters, which is used in JDBC to named parameters (e.g. select * from foo where id=? must be translated to select * from foo where id=@p1).
3. We want to support multiple statements separated by semi-colons in a single call to Connection#execute(String). This will allow clients to send statements like for example set statement_tag='foo'; select * from foo;. This significantly simplifies the use of features like tagging when using JDBC.

[olavloite]

All the methods that are added here, are only copy-pasted from PostgreSQLStatementParser. We do this as step 1 for a larger refactoring, so the next steps are easier to follow and review.

[olavloite]

This and the other methods here that appear to be deleted, are just moved to AbstractStatementParser, which is the super class of this class. That means that functionally, nothing changes.