Bug 867362 - part 1 - use MOZ_ALIGNOF instead of JS_ALIGN_OF_POINTER; r=luke

This change means that we no longer need the MOZ_ALIGN_OF_TYPE macro in
moznbytetype.m4.  And it's just nicer to have the compiler figure things
out for us rather than configury.
Bug 867362 - part 2 - delete JS_BITS_PER_DOUBLE; r=luke

Nothing in the engine uses it.
Bug 867362 - part 3 - delete JS_BYTES_PER_DOUBLE; r=luke

JS_BYTES_PER_DOUBLE isn't used after getting rid of JS_BITS_PER_DOUBLE.
Bug 867401 - don't generate checksums for saved pings; r=vladan

Generating checksums means relying on nsICryptoHash, which is
unavailable after profile-before-change.  We'd like to save pings after
profile-before-change.  Hence, don't checksum.  Over 99.99% of our pings
are read successfully across all channels, so checksumming isn't really
buying us anything.