Merge #12159: Use the character based overload for std::string::find.

Summary:
a73aab7 Use the character based overload for std::string::find. (Alin Rus)

Pull request description:

std::string::find has a character based overload as can be seen here
(4th oveload): http://www.cplusplus.com/reference/string/string/find/

Use that instead of constantly allocating temporary strings.

Tree-SHA512: dc7684b1551e6d779eb989e9a74363f9b978059a7c0f3db09d01744c7e6452961f9e671173265e71efff27afbcb80c0fe2c11b6dff2290e54a49193fa25a5679

Backport of Core PR12159
https://github.com/bitcoin/bitcoin/pull/12159/

Test Plan:

make check

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: deadalnix, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3750