Add snappy compression to drmemtrace output

Xref https://github.com/DynamoRIO/dynamorio/issues/2001#issue-175552118 where zlib with Z_BEST_SPEED was tried on the memtrace_x86 sample and was faster than writing uncompressed to an HDD but slower than an SSD (which was why it was not committed, though maybe it should have been under an option).

Xref #790 where general write-compressed-file support for all clients is requested.

This issue is about adding snappy compression for the direct output of drmemtrace. Snappy should out-perform zlib Z_BEST_SPEED (though with worse compression) and should be a net win for an SSD as well.

This issue can also cover generalizing this snappy compression into a library and to use it by default in the tracing samples.

In addition to improving overall performance, this will also shrink the disk space needed to store raw files for offline traces.

For online traces: this still should be a net win, shrinking the pipe throughput. We already have an offline file reader for snappy; we'd need to use that for the online reader.