Extract request creation procedure from box_process

 * Parse request type inside of request_create
 * Remove palloc from request_create
 * Replace request_execute with a fun pointer in struct request
 * Refactor box_process API to accept struct request *
 * Create requests directly in iproto.cc and box_lua.cc
 * Update memcached, recovery and other parts to use new API
Merge branch 'request-refactoring'