add faster checksum algorithm

[westram]

• add as additional algorithm (e.g. GBS_fast_checksum)
• use GBS_fast_checksum for checksum calculation in PT-Server and when calculating checksum which is sent to PT-Server
• other calls to GBS_checksum:
  • keep if result was stored in (e.g. acc generated by ARB or ACI command)
  • locations where checksum is only used temporarily (e.g. to control whether sequence was changed by mistake) can use GBS_fast_checksum

[westram]

• use ​http://en.wikipedia.org/wiki/Adler-32 for GBS_fast_checksum?

[epruesse]

SSE 4.2 added a CRC32C instruction, so I'm pretty sure that this will be faster than Adler-32 on modern CPUs.

It'd probably be best to leave this to a lib that deals with picking the proper implementation for a given CPU (and is updated as new CPUs come out). gcrypt and crcutil might be candates.