read_only and temporary spaces

[pgulutzan]

Read-only mode presumably exists to prevent persistent writes.
However, a change to a temporary space should not involve
the binlog anyway. Therefore it seems unnecessary to prevent
insert+update+delete on a temporary space. But it happens.

How to repeat:

tarantool> box.schema.space.create('Z',{temporary=true})
2016-03-10 10:22:52.273 [31701] wal/101/main I> creating `./00000000000000000062.xlog.inprogress'

---
- index: []
  on_replace: 'function: 0x40bc11a0'
  temporary: true
  id: 520
  engine: memtx
  enabled: false
  name: Z
  field_count: 0
- created
...

tarantool> box.cfg{read_only=true}
2016-03-10 10:23:10.505 [31701] main/101/interactive I> set 'read_only' configuration option to true

---
...

tarantool> box.space.Z:insert{1}
2016-03-10 10:23:26.113 [31701] main/101/interactive box.cc:112 E> ER_READONLY: Can't modify data because this server is in read-only mode.

---
- error: Can't modify data because this server is in read-only mode.
...

[rtsisyk]

Needed for #1821, moved to 1.7.3