allow using tcp keepalives instead of heartbeats

[tanmaykm]

AMQP allows heartbeats to be disabled completely, provided the server allows that. But it is essential to have an alternative to detect network disconnections and stale connections. In this PR, we switch on TCP keepalives on the connection (optional but enabled by default) and allow heartbeats to be switched off. Heartbeats are still enabled by default though.

The connection method has these new parameters to allow that:

connection(;
    heartbeat = true,
    keepalive = DEFAULT_KEEPALIVE_SECS,
)

• heartbeat: true to enable heartbeat, false to disable. Can also be set to a positive integer, in which case it is the heartbeat interval in seconds. Defaults to true. If false, ensure keepalive is enabled to detect dead connections. This parameter is negotiated with the server.
• keepalive: true to enable TCP keepalives, false to disable. Can also be set to a positive integer, in which case it is the keepalive interval in seconds. Defaults to DEFAULT_KEEPALIVE_SECS.

[codecov-commenter]

Codecov Report

Merging #47 (655ac4a) into master (6145773) will decrease coverage by 0.16%. The diff coverage is 94.11%.

@@            Coverage Diff             @@
##           master      #47      +/-   ##
==========================================
- Coverage   82.18%   82.02%   -0.17%     
==========================================
  Files           9        9              
  Lines        1016     1029      +13     
==========================================
+ Hits          835      844       +9     
- Misses        181      185       +4     

Impacted Files	Coverage Δ
src/protocol.jl	86.83% <94.11%> (-0.21%)	:arrow_down:
src/types.jl	96.31% <0.00%> (-0.60%)	:arrow_down:

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