FIB: optimise for src memory allocations

FIB: optimise for src memory allocations

Most FIB entries will only ever have one source providing forwarding
information. Currently the source infom is stored in a vector of sources
on the FIB entry. Change this to a union of one source inline and a vector.
This saves the need to alloc a vector of sources for each FIB entry.

before:
vpp# ip route add count 1500000 1.0.0.1/32 via 10.10.10.2 loop0
4.392857e5 routes/sec
vpp# ip route del count 1500000 1.0.0.1/32 via 10.10.10.2 loop0
9.175464e5 routes/sec
vpp# ip route add count 1500000 1.0.0.1/32 via 10.10.10.2 loop0
5.193375e5 routes/sec
vpp# sh fib mem
FIB memory
             Name               Size  in-use /allocated   totals
             Entry               72   1500011/ 1500011    108000792/108000792
         Entry Source            32   1500011/ 1500011    48000352/48000352

after:
vpp# ip route add count 1500000 1.0.0.1/32 via 10.10.10.2 loop0
4.726560e5 routes/sec
vpp# ip route del count 1500000 1.0.0.1/32 via 10.10.10.2 loop0
1.041629e6 routes/sec
vpp# ip route add count 1500000 1.0.0.1/32 via 10.10.10.2 loop0
5.702895e5 routes/sec
vpp# sh fib mem
FIB memory
             Name               Size  in-use /allocated   totals
             Entry               96   1500011/ 1500011    144001056/144001056
         Entry Source            32      0   /    0       0/0

Change-Id: Ic71e413eaff1ec152656beda3b94186f7894ea49
Signed-off-by: Neale Ranns <nranns@cisco.com>