sql: add cache statistics to box.info

To track current memory occupied by prepared statements and number of
them, let's extend box.info submodule with .sql statistics: now it
contains current total size of prepared statements and their count.

@TarantoolBot document
Title: Prepared statements in SQL

Now it is possible to prepare (i.e. compile into byte-code and save to
the cache) statement and execute it several times. Mechanism is similar
to ones in other DBs. Prepared statement is identified by numeric
ID, which are returned alongside with prepared statement handle.
Note that they are not sequential and represent value of hash function
applied to the string containing original SQL request.
Prepared statement holder is shared among all sessions. However, session
has access only to statements which have been prepared in scope of it.
There's no eviction policy like in any cache; to remove statement from
holder explicit unprepare request is required. Alternatively, session's
disconnect also removes statements from holder.
Several sessions can share one prepared statement, which will be
destroyed when all related sessions are disconnected or send unprepare
request. Memory limit for prepared statements is adjusted by
box.cfg{sql_cache_size} handle (can be set dynamically;

Any DDL operation leads to expiration of all prepared statements: they
should be manually removed or re-prepared.
Prepared statements are available in local mode (i.e. via box.prepare()
function) and are supported in IProto protocol. In the latter case
next IProto keys are used to make up/receive requests/responses:
IPROTO_PREPARE - new IProto command; key is 0x13. It can be sent with
one of two mandatory keys: IPROTO_SQL_TEXT (0x40 and assumes string value)
or IPROTO_STMT_ID (0x43 and assumes integer value). Depending on body it
means to prepare or unprepare SQL statement: IPROTO_SQL_TEXT implies prepare
request, meanwhile IPROTO_STMT_ID - unprepare;
IPROTO_BIND_METADATA (0x33 and contains parameters metadata of type map)
and IPROTO_BIND_COUNT (0x34 and corresponds to the count of parameters to
be bound) are response keys. They are mandatory members of result of
IPROTO_PREPARE execution.

To track statistics of used memory and number of currently prepared
statements, box.info is extended with SQL statistics:

box.info:sql().cache.stmt_count - number of prepared statements;
box.info:sql().cache.size - size of occupied by prepared statements memory.

Typical workflow with prepared statements is following:

s = box.prepare("SELECT * FROM t WHERE id = ?;")
s:execute({1}) or box.execute(s.sql_str, {1})
s:execute({2}) or box.execute(s.sql_str, {2})
s:unprepare() or box.unprepare(s.query_id)

Structure of object is following (member : type):

- stmt_id: integer
  execute: function
  params: map [name : string, type : integer]
  unprepare: function
  metadata: map [name : string, type : integer]
  param_count: integer
...

In terms of remote connection:

cn = netbox:connect(addr)
s = cn:prepare("SELECT * FROM t WHERE id = ?;")
cn:execute(s.sql_str, {1})
cn:unprepare(s.query_id)

Closes #2592