changed magic_value to from uint32_t to function.

So any tools compiling against libbitcoin will use the magic_values and the network compiled in, rather than having to define ENABLE_TESTNET. This also allows us to detect which network the installed libbitcoin is compiled for.
Support bootstrapping of testnet and faster bootstrapping using bootstrap-fast, which does not check the transactions inside blocks. If they can be deserialized they are considered valid and added to the chain.
Added detect-network to tools, to detect which network the libbitcoin.so is compiled for.