Merge pull request #474 from kanoi/master

cgsem - increase information on failure
Use a nonce2 offset variable for both gbt and stratum to consolidate requirements on work generation.
Rename pool gbt_coinbase variable to coinbase to combine it with the stratum coinbase data.
Reuse just the one pool coinbase variable in stratum, avoiding more string functions and storage in gen_stratum_work on each work generation.
Perform merkle bin hex2bin on stratum notify to avoid doing it on each work generation.
Rename work gbt_coinbase to coinbase to be in line with pool variable name.
Avoid one more hex2bin in gen stratum work.
Avoid a potential overflow should a pool specify a large nonce2 length with stratum.
Export the workpadding char in miner.h
Create a work data template when receiving stratum notification, allowing a simple memcpy of the merkle root avoiding more hex2bin conversions on each work generation.
Only perform the bin2hex on nonce2 data if it's required for stratum submission, thereby removing the last conversion of that type from stratum work generation.
Update NEWS for upcoming version 3.3.3
Bump version to 3.3.3