Garbage collection of old generations

[Rulexec]

• Add first_generaion_id field to collection
• Initially it is equal to generation_id
• In background we should watch for the readers in all collections and set first_generation_id to minimal generation_id of the readers
• User can create reader only with generation_id >= first_generation_id
• In background read all record keys of generations < first_generation_id, delete all outdated records (for which newer generation_id record present)
• If there is single record with generation_id < first_generation_id, copy this record with first_generation_id, then delete previous one
• Delete generations < first_generation_id

[Rulexec]

Done with ea9e196