investigate possiblilty of calculating EncryptedPartEntity content length

[cburroughs]

In EncryptedPartEntity we always return UNKNOWN_LENGTH for the content length. This puts us in chunked mode and may reduce performance by 10-30% (per @dekobon). It may be possible (in some cases) to calculate the content length, but this would have to account for the possibility of HMAC's and block boundaries.