Implement caching of PostgreSql statements

After the recent improvement regarding limitation of max connected users ( #48 (closed)), there is still existing one thing that can help to improve performance a bit.
Currently, each new user request requires to build set of PostgreSql statements. Of course they are already cached on the PqLib side, library returns code PG_42P05 (already prepared), but based on benchmark results it's not absolutely cheap.

I think that make sense to cache them also on the application side.
Preliminary dev plan:

• Introduce map of Statement objects in the Connection.
• Method Connection::prepare should return Statement object (build new or get from map).
• Refactor all existing usages of prepared statements

Update 01 Sep:

• Currently each new prepared statement has new name (can lead to degradation performance in time).

void Statement::Impl::buildStatementName() {
    auto uid = oes::core::Guid::make();
    std::stringstream ss;
    ss << std::this_thread::get_id();
    _statementName = "OES_" + uid.toStringNoDash() + "_" + ss.str();
}

• It is necessary to make deterministic generation names of statements.
• Need to avoid recreating statements with the same name (this will lead to abort transactions).