Postgresql: support schema "discovery" via search_path

Currently

• PostgresGenerator always qualifies table names et al with a schema.
• If the migration expression doesn’t explicitly specify a schema name via .InSchema, .WithSchema, etc then public is used

This bypasses search_path (eg when set in the Npgsql connection string) & so doesn’t allow schema to be set in config rather than code.

Support a configuration option, off by default, which when on affects handling of migration expressions without an explicit schema as follows:

• Existence tests in PostgresProcessor query all schemas in search_path
• SQL generation in PostgresGenerator doesn’t qualify names with a schema

My use-case for this:

• I have 15 existing services using FluentMigrator & Postgres, with a database for each service
• I want to migrate the data into a single database with a schema for each service
• I’d like to do this by setting search_path for each service, avoiding per-service code changes

[fubar-coder]

Instantiate a new convention set:

var conventionSet = new DefaultConventionSet(defaultSchemaName, null);

Append it to your services (after the AddFluentMigratorCore):

services.AddSingleton<IConventionSet>(conventionSet);

[jzabroski]

@i-sinister I moved the above table and footnotes to: https://github.com/fluentmigrator/fluentmigrator/blob/master/adr/proposed/DefaultSchema.md