increase performance of bmv2 / simple_switch

[antoninbas]

Even though this is not the primary design goal of bmv2, the following ideas can be investigated:

1. use a stack allocator in appropriate cases for class ByteContainer
2. use a custom allocator for libgmp, could also speed up the phv::reset() operation used by simple_switch
3. investigate replacement for libpcap to speed up I/O
4. cache header and field ids in simple_switch instead of always looking up fields by name
5. exact match cache for ternary match lookup

[antoninbas]

#112 implements 1.