Add support for mmap-backed CDB reader (yielding a x35 improvement)

[cipriancraciun]

Currently the CDB reader uses io.ReaderAt, that for each read operation will issue a read system call (or equivalent). An alternative would be to use mmap-ed memory to access the backing file.

In my experiments (see the patch below) by using mmap yields a 35 times performance improvement:

BenchmarkGet-4          13902710              2571 ns/op              28 B/op          2 allocs/op
BenchmarkGetMmap-4      484713384               73.96 ns/op            0 B/op          0 allocs/op

The following are a few patches that provide such support. If accepted I can submit a pull request:

• cipriancraciun@6c948e5 -- adds support of using a []byte buffer that backs the CDB reader; such a []byte buffer can be either the result of a mmap or just reading the entire file in memory;
• cipriancraciun@0b1bb73 -- this adds support for a custom io.Close to be called on cdb.Close; (it will be used to munmap the memory if needed;)
• cipriancraciun@924564b -- this adds the actual mmap support;

The only semantic change is the following:

• the []byte value returned as result of Get should not be written to by the application, as it's actually a slice from the mmap-ed file;
• the []byte value can't be used after the cdb.Close was called;
• if this is not acceptable, and one still wants to keep the current (undocumented) semantic, one could clone that slice (thus incurring an extra alloc, which in my patch doesn't happen);