Context isn't freed in the ECDH benchmark

Summary:

• free the ctx at the end of bench_ecdh
• Pass num of iters to benchmarks as variable, and define envvar
• Add running benchmarks regularly and under valgrind in travis

This is a backport of libsecp256k1 PR722

Test Plan:

ninja all check check-secp256k1 bench-secp256k1

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6364