Rename New SQL schema to Multihost, and Default to Singlehost

[badlop]

When ejabberd first got support for SQL storage, it only supported one vhost, so it made sense to not store the host in the SQL tables. Additionally, the SQL schema in ejabberd followed that of jabberd14, which didn't support vhost either.

When ejabberd got support for multiple vhosts, if several of them want to use SQL storage, the solution is to configure a different SQL database for each vhost using the host_config toplevel option.

However, when there are many vhosts configured in ejabberd, all of them using SQL storage, it is preferable to setup one single SQL database, and store the vhost in the tables. When that feature was added to ejabberd, it got the name of "new SQL schema". And the previous SQL schema was called "legacy", "old", and nowadays "default".

The problem with the terms "default" and "new" is that they are circumstantial, and do not really describe the schema features or purposes.

I propose to rename:

• "default SQL schema" --> "singlehost SQL schema"
• "new SQL schema" --> "multihost SQL schema"

---

Right now all names are supported, the previous (obsolete) and the renamed (preferred). The changes relevant to the usage are:

When preparing configuration, the arguments:

  ./configure --enable-new-sql-schema
  ./configure --enable-multihost-sql-schema

When configuring ejabberd, the toplevel options:

  new_sql_schema: true
  sql_schema_multihost: true

When developing source code, the functions:

  ejabberd_sql:use_new_schema()
  ejabberd_sql:use_multihost_schema()

[coveralls]

coverage: 33.815%. remained the same
when pulling 5575d4e on badlop:multihost
into 4dea2f1 on processone:master.