box: implement auto increment

The value returned by space.auto_increment() is not guaranteed to be
monotonically growing, because this method uses max+1 for the next
generated value, which can be less than a value generated before in case
the max key is deleted. Besides, calling max() may be expensive, e.g. it
requires a disk access in case of Vinyl. So this patch implements auto
increment based on persistent sequences.

To be used for auto increment, a sequence needs to be attached to a
space. To attach an existing sequence to a space, pass 'sequence=ID' in
index options when creating or modifying the primary key. Here ID is the
name or the numeric identifier of the sequence to attach. Note,
sequences can be attached only to spaces with integer primary key.
After a sequence is attached, it will be used for auto increment when
the user passes 'nil' for the primary key value. If the user passes a
value different from 'nil', the value will be used to update the
attached sequence.

To detach a sequence from the space it was attached to, either drop the
primary key of the space or alter the primary key with 'sequence=false'.

It is not necessary to pre-create a sequence to use it for auto
increment - a sequence can be automatically generated. To generate a
sequence automatically, pass 'sequence=true' in index options.
Automatically generated sequences are named '_auto_ID', where ID is the
space id, and removed when the space is dropped or the sequence is
detached.

A sequence cannot be dropped as long as it is attached to any spaces.

Closes #389