SerializedSnapshotData alignment

Dear 7b4ac19 authors,

Field ps_snapshot_data usually receives four-byte alignment within
ParallelIndexScanDescData, but it contains the eight-byte whenTaken field.
The select_parallel test dies with SIGBUS on "Oracle Solaris 10 1/13
s10s_u11wos_24a SPARC", building with gcc 4.9.2. Some credible fixes:

1. Move the SerializedSnapshotData declaration from snapmgr.c to snapmgr.h and
declare the ps_snapshot_data field to be of type SerializedSnapshotData.
Probably also add a field "TransactionId xids[FLEXIBLE_ARRAY_MEMBER]" to
SerializedSnapshotData, to assert the variable-length nature.

2. Change "char ps_snapshot_data[FLEXIBLE_ARRAY_MEMBER]" to "int64 ...". I
have attached this in SerializedSnapshot-int64-v1.patch.

3. Change no declarations, and make snapmgr.c memcpy() the
SerializedSnapshotData through a local buffer. I have attached this as
SerializedSnapshot-memcpy-v1.patch.

I like (2) well enough, but I don't see that technique used elsewhere in the
tree. (1) is more typical of PostgreSQL, though I personally like it when
structs can stay private to a file. (3) is also well-attested, particularly
in xlog replay code. I am leaning toward (2). Other opinions?

Field phs_snapshot_data happens to receive eight-byte alignment within
ParallelHeapScanDescData; otherwise, such scans would fail the same way.