Fix max call in keys manager

[lebdron]

Description of the Change

Fix type incompatibility on 32-bit platforms (armv7, Windows build for x86) which caused build error.

Benefits

It compiles without errors on 32-bit platforms.

Possible Drawbacks

None.

[bakhtin]

Successfully compiled binaries after applying this patch on ARMv7 platform:

root@d29676a564e0:/opt/iroha_deps/iroha# file build/bin/iroha-cli 
build/bin/iroha-cli: ELF 32-bit LSB shared object, ARM, EABI5 version 1 (GNU/Linux), dynamically linked, interpreter /lib/ld-linux-armhf.so.3, for GNU/Linux 3.2.0, BuildID[sha1]=9d4847e33995a30e125b71a6555830cc7bc4fa91, not stripped
root@d29676a564e0:/opt/iroha_deps/iroha# file build/bin/irohad    
build/bin/irohad: ELF 32-bit LSB shared object, ARM, EABI5 version 1 (GNU/Linux), dynamically linked, interpreter /lib/ld-linux-armhf.so.3, for GNU/Linux 3.2.0, BuildID[sha1]=5cd9ab5b37008b1b79edb6475acfec8a0abe9cb7, with debug_info, not stripped