sql: introduce holder for prepared statemets

This patch introduces holder (as data structure) to handle prepared
statements and a set of interface functions (insert, delete, find) to
operate on it. Holder under the hood is implemented as a global hash
(keys are values of hash function applied to the original string containing
SQL query; values are pointer to wrappers around compiled VDBE objects) and
GC queue. Each entry in hash has reference counter. When it reaches 0
value, entry is moved to GC queue. In case prepared statements holder is
out of memory, it launches GC process: each entry in GC queue is deleted
and all resources are released. Such approach allows to avoid workload
spikes on session's disconnect (since on such event all statements must
be deallocated).
Each session is extended with local hash to map statement ids available
for it. That is, session is allowed to execute and deallocate only
statements which are previously prepared in scope of this session.
On the other hand, global hash makes it possible to share same prepared
statement object among different sessions.
Size of cache is regulated via box.cfg{sql_cache_size} parameter.

Part of #2592