hrpc: Count all cellblocks in valuesToCellblocks
hrpc: Count all cellblocks in valuesToCellblocks

Preallocate room for all cellblocks

This results in 20% fewer allocations when serializing large multi
requests:

name                             old time/op    new time/op    delta
MultiToProto/cellblocks-16         5.25µs ± 0%    5.47µs ± 0%   +4.26%  (p=0.000 n=10+9)
MultiToProto/no_cellblocks-16      5.05µs ± 0%    5.03µs ± 0%   -0.38%  (p=0.002 n=8+9)
MultiToProtoLarge/cellblocks-16    2.19ms ± 0%    1.80ms ± 1%  -17.76%  (p=0.000 n=8+10)

name                             old alloc/op   new alloc/op   delta
MultiToProto/cellblocks-16         3.42kB ± 0%    3.42kB ± 0%     ~     (p=1.000 n=10+10)
MultiToProto/no_cellblocks-16      3.84kB ± 0%    3.84kB ± 0%     ~     (all equal)
MultiToProtoLarge/cellblocks-16    1.99MB ± 0%    1.76MB ± 0%  -11.63%  (p=0.000 n=10+10)

name                             old allocs/op  new allocs/op  delta
MultiToProto/cellblocks-16           58.0 ± 0%      58.0 ± 0%     ~     (all equal)
MultiToProto/no_cellblocks-16        64.0 ± 0%      64.0 ± 0%     ~     (all equal)
MultiToProtoLarge/cellblocks-16     15.2k ± 0%     12.2k ± 0%  -19.75%  (p=0.000 n=10+10)

tsuna/gohbase