Nuking db during development

[groue]

This is a pull request that addresses #378 by @bellebethcooper.

---

A DatabaseMigrator can automatically wipe out the full database content, and recreate the whole database from scratch, if it detects that a migration has changed its definition:

var migrator = DatabaseMigrator()
migrator.eraseDatabaseOnSchemaChange = true

Beware! This flag can destroy your precious users' data!

Yet it may be useful in those two situations:

1. During application development, as you are still designing migrations, and the schema changes often.

   In this case, it is recommended that this flag does not ship in the distributed application:

   var migrator = DatabaseMigrator()
   #if DEBUG
   // Speed up development by nuking the database when migrations change
   migrator.eraseDatabaseOnSchemaChange = true
   #endif

2. When the database content can easily be recreated, such as a cache for some downloaded data.

The eraseDatabaseOnSchemaChange option triggers a recreation of the database if the migrator detects a schema change. A schema change is any difference in the sqlite_master table, which contains the SQL used to create database tables, indexes, triggers, and views.

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This pull request also adds the two following methods:

• DatabaseWriter.erase()
• DatabaseWriter.vacuum()

Those are not documented in the main README, and only exposed in the reference documentation.

[groue]

@bellebethcooper, what do you think of the description of the feature?

[bellebethcooper]

@groue This looks great! So easy to use, also. Thanks for the quick turnaround! I think this will be super useful in future :)

[groue]

Sounds great 👍 Thank you, and Realm, for the inspiration!

[groue]

Shipped in v3.2.0!