Thread-safety: replace buffers thread-safe pools

[lehugueni]

This PR replaces the temporary buffers stored in objects with thread-safe pools that can be used to obtain temporary objects.

• Added the structs.BufferPool interface and structs.SyncPool, structs.BuffFromUintPool that implement it.
• Modify ring.Ring so that it stores a structs.SyncPool[*[]uint64]
• Add methods that build pools of ring.Poly, ringqp.Poly and rlwe.Ciphertext from structs.SyncPool[*[]uint64]
• Replace buffers with these pools in all non-thread-safe structures of FHE:
  • rlwe.Evaluator
  • rlwe.Decryptor
  • ring.BasisExtender
  • ckks.Evaluator
  • ckks.Encoder
  • bgv.Evaluator
  • bgv.Encoder
  • sampler.Gaussian
  • sampler.Uniform
  • rlwe.Encryptor
  • rlwe.KeyGenerator
• Replace buffers with pools in all non-thread-safe structures of MHE
• Add tests for correctness of concurrent executions
• Remove ShallowCopy functions