Add and use set/get_uint64 on onion tags. [bug 604; backportable]

It seems that 64-bit Sparc Solaris demands 64-bit-aligned access to
uint64_t, but does not 64-bit-align the stack-allocated char array we
use for cpuworker tags.  So this patch adds a set/get_uint64 pair, and
uses them to access the conn_id field in the tag.

svn:r18743