[Perf] Build rocksdb with jemalloc enabled

[ljedrz]

Over time we have tested various performance-related RocksDB options that were recommended in the official documentation and different case studies compiled by its users, but most of the time the effect on snarkOS memory use hasn't been significant.

One of the greatest improvements wrt node RAM figures to date has been the switch from the OS allocator to jemalloc, which is much better at handling memory fragmentation and parallel workloads.

It appears, however, that in order to utilize jemalloc within RocksDB specifically, it must be built with the seemingly undocumented jemalloc feature, which this PR does.

Filing this as a draft, as there are 2 open points:

• we are currently testing it on a client in the IsoNet, but we'll also need to test it under higher load and with a validator
• jemalloc is currently included only for x86_86 Linux, so we may (on systems without jemalloc it might just get ignored) need to tweak some TOMLs in order for it to work on other operating systems

[vicsn]

Consider making jemalloc conditional on the architecture, just like our global jemalloc configuration