box: introduce prepared statements

This patch introduces local prepared statements. Support of prepared
statements in IProto protocol and netbox is added in the next patch.

Prepared statement is an opaque instance of SQL Virtual Machine. It can
be executed several times without necessity of query recompilation. To
achieve this one can use box.prepare(...) function. It takes string of
SQL query to be prepared; returns extended set of meta-information
including statement's ID, parameter's types and names, types and names
of columns of the resulting set, count of parameters to be bound.  Lua
object representing result of :prepare() invocation also features two
methods - :execute() and :unprepare(). They correspond to
box.execute(stmt.stmt_id) and box.unprepare(stmt.stmt_id), i.e.
automatically substitute string of prepared statement to be executed.
Statements are held in prepared statement cache - for details see
previous commit.  After schema changes all prepared statement located in
cache are considered to be expired - they must be re-prepared by
separate :prepare() call (or be invalidated with :unrepare()).

Two sessions can share one prepared statements. But in the current
implementation if statement is executed by one session, another is
not able to use it and will compile it from scratch and than execute.

SQL cache memory limit is regulated by box{sql_cache_size} which can be
set dynamically. However, it can be set to the value which is less than
the size of current free space in cache (since otherwise some statements
can disappear from cache).

Part of #2592