[services] Tunnelbroker - Fix warnings during the AmqpManager build

Summary:
Fixing small std::string conversion warnings during the build process:

AmqpManager.cpp:40:30: use of overloaded operator '=' is ambiguous (with operand types 'AMQP::AssociativeFieldProxy' (aka 'FieldProxy<AMQP::Table, basic_string<char, char_traits<char>, allocator<char>>>') and 'const size_t' (aka 'const unsigned long'))
[build]   arguments["x-message-ttl"] = AMQP_MESSAGE_TTL;

Test Plan: Run yarn run-tunnelbroker-service and successfully built the service without any warnings.

Reviewers: karol-bisztyga, palys-swm, varun

Reviewed By: karol-bisztyga, palys-swm

Subscribers: ashoat, palys-swm, Adrian, atul, karol-bisztyga, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3830