MbedTLS uses small internal buffers for encryption

[boryspoplawski]

Description of the problem

When using encrypted pipes (e.g. to send checkpoint) MbedTLS has to encrypt the data and uses 4 pages buffer for that. This might drastically reduce fork performance in case of big enclaves.

Proposed solution (needs performance improvement verification):
set MbedTLS buffer size (hopefully possible) to some grater value, probably tied to enclave size (e.g. we would have 3 different sizes depending on the enclave size).

[dimakuv]

Looks like a simple performance optimization, but someone will need to do thorough performance analysis of this change. Doesn't seem too important at the moment, so assigning low priority.